# [SOLVED] 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!



## Kevin350

Hello everyone,

I tried to make this easy to read and understand.

1990 Chevrolet C1500 -- 350 V8 TBI -- Regular Cab, Short Bed, 2 wheel drive, 89200 miles. 

*Bonus note:* Truck burns oil in #1 and #8 cylinders. Both spark plugs have extensions on them to prevent oil from fouling out the plugs. Spark plugs show carbon deposits, but not much.

*First Note: Just in case this helps* Two weeks ago my truck was stalling out at complete stops when the engine was at operating temperature (engine runs strong when driving on city roads and highway roads). I changed the fuel pump and it did not solve the problem. I changed the ignition control module and ignition coil and the truck ran great and it did not stall out like it did before the control module and ignition coil were replaced (so far so good).

*Second Note: Just in case this helps:* I failed my emissions test in June 2009. Engine put up 565 points for the (HC) category (to pass it needs to be under 160). I took it back for a re-test in July 2009 and passed the emissions test by 2 points for the (HC) category. (My truck scored 159 out of 160.) (I replaced the cap, rotor, and EGR valve to make the truck pass.)

Everything on truck is 19 years old, except: 

New parts: EGR valve, coolant temperature sensor, fuel filter, idle air control valve, pcv valve, air filter, 1 oxygen sensor (in the exhaust manifold), spark plugs, distributor cap and rotor.

*Here is my problem:* Today, I was on the highway doing about 65-70 mph for about 45 minutes and the "Service Engine Light" came on. I got home alright (it did not stall because when I came to stop signs and traffic lights I held the brake and kept the gas peddle down a certain amount to keep everything in the engine moving). In my driveway, I let the truck sit in complete idle (without pressing the gas pedal) for about 5 minutes. I can tell it wants to die out, but it never did. (Although, if I keep driving the way it is it probably will die out sooner or later.) It gets even worse when the A/C is on. You can tell that the idle air control valve is working very hard. The engine shakes rough enough to visibly see the truck shake from inside the cab. (This is the second time the service engine light came on while driving at highway speeds.) (Engine light came on before control module and coil were replaced and it did again today after the replacement of those two parts.)

*Third Note:* When accelerating it has a little hesitation to it. Also, when downshifting and mashing the throttle to get back up to speed, you can tell it hesitates very slightly like something is holding the engine back.

The first time my engine light came on I did not get a code. Today, after the service engine light came on, I got a code. Also, both times, when I turned the truck off and restarted the engine, the engine light did not come back on.

The code was -- 32 

Code 32 is: a) Barometric pressure sensor circuit failure. 

b) EGR valve diagnostic switch - closed during engine start up or open when egr flow requested by ecm. Or EGR/EVRV 

Is it a bad, 2 month old EGR valve? Or could it be something else?

Thank you,

-Kevin


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hi kevin 

Code 32 is really a stubborn problem on GM TBI's. In human terms the code means that there is no oxygen in the EGR gas, and for that reason the EGR works with what is known as the integrator or Block learn. The integrator counts how many times the EGR is opened and closed, this causes the MAP sensor to adjust the air\fuel mixture. The EGR works primarily with the MAP sensor, and depending on what reading the O2 sensor gives the computer, the MAP sensor compensates the air/fuel mixture. It is imperative that you check the MAP sensor readings with vacuum gage and a DVOM. Also check the vacuum lines for breaks or cracks from the manifold. IMO your MAP sensor needs to be replaced.
Also keep in mind the EGR's either work with positive pressure or negative pressure, you will have to look at the EGR to figure out which type it works with.

Here's a link how to show you what type of EGR you have : http://shbox.com/1/EGR_ID.jpg

post back your findings.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hello octaneman!

Thank you for replying. It is much appreciated.

I tested the EGR valve today. I disconnected the EGR valve and put a rubber hose from the EGR valve to the throttle body to see if the engine would run rougher. The engine stayed the same which means that the EGR valve is bad. (Please correct me if I'm wrong.) Also, I hooked up a manual vacuum tester to the EGR valve and manually pumped vacuum into the EGR valve. It still did not change anything which means that the EGR valve is not working. (Please correct me if I'm wrong.)

1. My EGR valve is aftermarket. It looks the same as the one in the picture you provided, but my EGR valve is gold on the top of the valve. It has a "N" for negative pressure just the like the one in the picture you provided.

When the EGR valve was taken out this morning, the bottom of the EGR valve where it screws in had carbon in it (the valve is only 2 months old). My truck, on cold starts in the morning, (after sitting all night) blows bits of carbon out of the exhaust. If I let it idle long enough, it will leave marks of carbon on the ground. This could be from the burning oil in #1 and #8 cylinders. Also, if I floor the throttle, sometimes you will see light black smoke go out the exhaust.

2. I cleared the code from the computer by disconnecting the negative terminal from the battery for 5 minutes.

3. I'll admit that I did not test the MAP sensor because I don't have a digital ohm meter. Would an analog ohm meter with the needle work to test the MAP sensor?

Instead of testing the MAP sensor using the meter, can I just disconnect the MAP sensor and if the engine runs better, then it is a bad MAP sensor?


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin 

If you disconnect the MAP sensor the computer will set a Map sensor code 33-34 and the engine will run worse or not at all. When the check engine light comes on it will set 2 types of codes. 1) hard code, 2) soft code. A hard code is when the computer registers a serious fault from the sensors and the check engine light remains on constantly. A soft code is when the check engine light goes on and off intermittently. Computer codes function and diagnosed by priority, the higher the number, the higher the priority, so by disconnecting the MAP sensor you will be setting yourself back to square 1, and yes you can test the MAP with an analogue meter. 

I find it difficult to believe a 2 month old EGR valve can go bad. When testing it with a vacuum gage, if the diaphragm holds vacuum then the EGR is still good even though the engine does not respond. This would mean that the orifice of the EGR (and where it sits) is carbonized, and you can attempt to clean it before going out to replace the EGR again. On the other hand, if the diaphragm does not hold vacuum then the EGR is bad even though it's a new one. Remember, the EGR has to hold vacuum, and the engine has to respond when opened. Since your engine is severely carbonized, I would suggest cleaning it out using sea-foam. If you decide to use sea-foam please follow specific directions on how to use it for your engine. 

You should know that black smoke coming from the exhaust means the engine is running rich, and blue smoke means that you are burning oil. The excessive black smoke from the exhaust leads me to suspect the MAP sensor cannot trim the air/fuel mixture due to the ERG problem. Clear out the EGR and it's ports from carbon build up ( if the diaphragm does not work replace it), then test the MAP sensor with your meter and see if it works properly.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

I have a 89 S-10 Blazer with a 4.3, same stuff as yours, just not as many cylinders

Code 32 is either EGR circuit on TBI baro circuit on Carburetor models, Yours will be EGR, if you manually fixed a hand vac pmp to the egr and nothing happened, yea the EGR is bad, OR the vac line is bad, or with the carbon on the plugs you might very well have a carbon buildup in the path blocking the egr gas remove the egr and look if its clogged, unclog it.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

I used my analogue meter to test the MAP sensor.

My meter is similar to this one (but the numbers on my scale where the needle points to are a little different): 

http://www.professionalequipment.com/enlargeproduct.asp?productid=10413

I'm guessing it is an old one.

I don't know how to use this analogue meter very well. So, just in case, I'm going to give you all the results from where the needle lined up in all of the categories. So, if you are having a bad day, then you are going to get a laugh out of this one.

I put my meter's dial at "12" in the "DCV" range.

I took the MAP sensor's electrical connector off and put the positive lead to the gray wire (3rd terminal) and I put the negative lead to the purple wire (1st terminal). I turned the ignition to the "ON" position. 

The results:

OHMS: 7
DC (1st row): 54
DC (2nd row): 25
DC (3rd row): In the middle of 10 and 15
AC (30V UP): I have no idea. It's in red but does not show any numbers above the lines.
AC (6 volt only): 2.5

Then, I reconnected the MAP sensor's connector and put the positive (red) lead in where the green wire goes into (2nd terminal). I put the negative (black) lead onto a bolt on the engine (for a good engine ground). I turned the ignition to "ON" position. 

I received the same results as above.

I fired the engine up. The needle went down....

OHMS: Between 100 and 40
DC (1st row): 10 (between 0 and 20)
DC (2nd row): 5 (between 0 and 10)
DC (3rd row): In the middle of 0 and 5
AC (30V UP): ?
AC (6V Only): 0.5

Or should I just start over again and buy a digital multimeter?

-------------------------------------------------------------------

Wolfen1086,

Thank you for replying!

The EGR valve was cleaned out. I also cleared the code from the computer. I will see if the code comes back. While waiting for the code to come back, I am testing or trying to test the MAP sensor to see if that's bad.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin350


After looking at the results of your tests, the second test you did with the harness connected to the MAP sensor are readings that are within range of specs. A serious point of interest is the reading : [ DC (3rd row): In the middle of 0 and 5 ]. When you obtained this reading was the engine at idle? or were you throttling up the engine ? This is critical, from your test to get an accurate diagnosis of the sensor. To accurately test the MAP sensor with an analogue meter, the needle of the meter must increase to 5volts DC as you throttle up the engine. Another key factor is when you accelerate check to see if the needle drops to zero, if it drops, another sign of a bad sensor. Always keep in mind when testing the sensor, that the voltage readings will change as engine load changes. This is the proper method for testing MAP sensors. Remember, the center wire on the MAP sensor is the one that goes to the computer, this is the wire where you connect the positive side of the meter. 

The second test on the MAP sensor is by applying vacuum to it and see if it holds it. Testing the MAP off the car would require you to put a power source to it in conjunction with a vacuum pump and your meter hooked up as I previously described. When applying vacuum the meter readings should change. If you decide to get a digital meter make sure that there is a frequency scale. You will use the same procedure with the digital meter as you did with the analogue, the only difference is you will perform 3 tests instead of 2. When testing sensors, the frequency scale is the one used religiously to record readings very accurately, and pinpointing trouble spots.

Please re-post your procedure when you got this ---> DC (3rd row): In the middle of 0 and 5

post back your findings.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin it looks like you did ok with a analog meter, Yea a digital is way better, but if you get steady needle readings on the analog and your reading the right row of numbers, then your in good shape on using the meter, as for the numbers, I can't tell you if those numbers mean good or bad though, my book on Chevy full size stops at 1977, so my manual covers only carbs with no emission sensors at all, except the ERG valve itself.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

I re-tested the MAP sensor. Same procedures as the above post with analogue meter. All results are taken from DC (3rd row).

First test with the ignition key "ON" and positive lead going to gray wire and negative lead to purple wire:

*Results:* DC (3rd row): Between 10 and 15

Second test with ignition key "ON" and positive lead to middle wire of map sensor (green wire) and negative lead to ground (one of the bolts on the frame rails):

*Results:* DC (3rd row): Between 10 and 15.

Same test (green wire, engine ground), but with engine in idle:

*Results:* Between 0 and 5 (2.5)

Same test (green wire, engine ground), but throttling up hard:

*Results:* It goes from 2.5 to 5 volts.

Now, here are the results from the vacuum test:

With the ignition key turned to "ON" (engine off), I took the wire from the carburetor (?) off of the MAP sensor and I hooked up my manual vacuum pump. I pumped vacuum into the MAP sensor. My analogue meter's positive lead was still in the middle wire (green wire) and negative lead was on engine ground.

*Results: *

The volts started between 10 and 15. Then with one pump of vacuum it went down to 10, then one more pump it went down in between 5 and 10, then another pump it went down to 5. Then another pump it went down to 2.5. And the needle stayed there.

The vacuum gauge read: 25


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



Kevin350 said:


> Octaneman,
> 
> I re-tested the MAP sensor. Same procedures as the above post with analogue meter. All results are taken from DC (3rd row).
> 
> First test with the ignition key "ON" and positive lead going to gray wire and negative lead to purple wire:
> 
> *Results:* DC (3rd row): Between 10 and 15
> 
> Second test with ignition key "ON" and positive lead to middle wire of map sensor (green wire) and negative lead to ground (one of the bolts on the frame rails):
> 
> *Results:* DC (3rd row): Between 10 and 15.
> 
> Same test (green wire, engine ground), but with engine in idle:
> 
> *Results:* Between 0 and 5 (2.5)
> 
> Same test (green wire, engine ground), but throttling up hard:
> 
> *Results:* It goes from 2.5 to 5 volts.
> 
> Now, here are the results from the vacuum test:
> 
> With the ignition key turned to "ON" (engine off), I took the wire from the carburetor (?) off of the MAP sensor and I hooked up my manual vacuum pump. I pumped vacuum into the MAP sensor. My analogue meter's positive lead was still in the middle wire (green wire) and negative lead was on engine ground.
> 
> *Results: *
> 
> The volts started between 10 and 15. Then with one pump of vacuum it went down to 10, then one more pump it went down in between 5 and 10, then another pump it went down to 5. Then another pump it went down to 2.5. And the needle stayed there.
> 
> The vacuum gage read: 25




The test results you posted clearly shows that the MAP sensor is functioning, the most compelling evidence of this is when you throttled up hard and the voltage increased to 5v. The vacuum test you did also reinforces that conclusion when you applied 25Hg and the voltage changed. On a previous post you mentioned that you were not proficient with an anologue meter, on the contrary,judging from the results you posted you're batting 1000... Nicely Done!


This also means that the annoying code 32 will still pop up and the problem still remains. The problem now is rendered academic, all of your attention must be focused on the EGR gas pathway and the O2 sensor. The EGR works with engine vacuum, the O2 sensor tells the computer that it's too rich\lean giving the signal to the MAP to trim out the fuel ratio. This is not happening due to carbon build up. Carbon build up wreaks havoc with sensors, by visually inspecting and testing the O2 sensor, it can show just how much carbon has been a factor, then the carbon must be flushed out.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

I appreciate your replies! You have been very helpful. Without your help, I wouldn't have any idea where to start. I would have been throwing parts at the truck. And I'm learning which is good.

Before I posted the results, I had a bad feeling that you were going to say something about the O2 sensor in your next post. And, you did post about the O2 sensor and the severely carbonized engine that I have.

The reason is because I changed that O2 sensor (last year?) and it was black with carbon. I'm pretty sure that this O2 sensor is black too from all the carbon. 

Three things that I can think of to get rid of the carbon:

a) Seafoam -- you mentioned this to me. I have a throttle body injection. I will do this and see what happens, but I realize that it will be a temporary fix. 33% of the Seafoam can in the intake manifold, 33% into the fuel tank, and the rest of the can into the oil pan (obviously through the hole where I put my oil). Would this be correct? If so, what would be the best way to put the Seafoam into the intake manifold? Any vacuum line going to the intake manifold?

b) New valve seals/guides -- my truck burns oil in the #1 and #8 cylinders. And the burning of the oil is creating all of this carbon. I had new valve seals done two years ago by a mechanic. It didn't solve the oil burning problem. (Although, at that time, only #1 cylinder was burning oil -- two years later my #8 cylinder started burning oil.) (Also, in the past two years, my truck hesitated for a couple months in idle and acceleration and then it cleared up. It ran excellent for another couple months and then it went back to hesitating again.) (Now, it hasn't cleared up in idle or acceleration for 6-10 months now which means the problem has gotten worse.) The mechanic checked the valve seal job that he did again to make sure they were in place. He said everything looks fine. I always question it though. Either the new valve seals slipped off or the problem is the piston rings. I do remember driving it down the street and a big cloud of black smoke came out the exhaust after the mechanic did the valve seal job on all eight cylinders. With my luck, it is bad piston rings.

c) New piston rings -- it's not even worth going through the trouble. I might as well keep driving the truck the way it is right into the ground.

Also, what do you think of a new distributor pickup coil? I heard they are common to give engine's hesitation problems? I already changed the ignition control module and ignition coil. How about the pickup coil? Or should I leave it alone and face the fact that my truck will never run right until the carbon is cleaned up?


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin

The best method in running seafaom to the intake manifold is to disconnect the vacuum line from the brake booster and feed it from there. The proportions you mentioned to distribute the seafoam throughout the engine are correct. Disconnect the fuel pump fuse and start the engine with only seafoam. This will maximize the cleaning action throughout the intake manifold and it will break any deposits still inside. It is like using injector system flush without the complex set-up. Bear in mind that when you do this, the engine has to be throttled up at a high rpm for it to take effect. Please take safety precautions by wearing a mask to reduce inhaling the toxic fumes and be in a well ventilated area, the exhaust smoke will build up extremely fast. Do not run seafoam inside the crank case for a long period, seafoam will brake down the oil's viscosity very quickly, so after it's done an oil change must be done immediately after running it. Make sure that you flush out the intake manifold first before adding it to the crankcase and gas tank.

Excessive black smoke coming from the exhaust means that the engine is running very rich, and it is a clear indication of a massive vacuum leak. It may be coming from either the TBI base gasket, the intake manifold, or the brake booster. If you had said that the exhaust fumes are blue, then I would agree with you that that your engine is burning oil, but so far it is only black smoke thats coming from the exhaust. The best way to check for leaks is with a vacuum gage hooked up to the engine, spraying carb cleaner or water on the intake does not always pinpoint to where the problem is. The vacuum gage readings will leave no doubt that valves are either leaking or something is seriously wrong with the intake manifold. 

To check for oil deposits, remove the spark plug and see if it is wet from oil, also check the PCV valve for oil contamination. If the spark plug is saturated, then the piston rings are worn out, then it is better to just run the engine as it is until it dies, just add oil periodically because rebuilding an engine today just isn't worth the cost.

To answer your pickup coil question just use the old fashioned rule of thumb which states: If It isn't Broken Don't Fix It. There are faults that do come from the electrical system, but when that does happens the engine will either stumble or just die out completely with no warning. It is a good idea to just leave it alone and focus on how to clear out the carbon problem.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

I will use your advice when I use the Seafoam. I will also change the O2 sensor and put in a new one after the Seafoam does its job.

As for vacuum leaks, I don't hear any sucking air from anywhere within the engine compartment. I would like to do a full vacuum test on my engine though to make sure. Do I need to hook up my manual vacuum pump to all of the vacuum lines and test each of them or can I only test one of them? When I hook my vacuum pump and gauge to the vacuum hose(s), do I give it one pump and if the engine sounds like it is going to stall, then that means there are no vacuum leaks coming from the hose(s)?

You mention the vacuum leak could be coming from the brake booster. I don't know if this affects anything, but my brake pedal (even with new brakes) is very soft (common problem with my model). New brakes and master cylinder were installed several months ago, bled the brake system well (with no leaks from master cylinder), and after a couple of weeks the brake pedal would get softer and softer. My pedal doesn't go all the way down to the floor, but it sure feels like it wants to. After 19 years, could my brake booster be bad which would cause soft brakes *AND* vacuum leak problems?

Octaneman, I have all different colors coming out of my exhaust. I have blue smoke, black smoke, and very fine/light black/gray smoke. (No white smoke, knock on wood.) I realize black smoke means rich fuel mixture, but could the black smoke *only* be created by the severely carbonized engine? Can the black smoke be created by the carbonized engine and rich fuel mixture together? Or can black smoke only be created by rich fuel mixture?

Three years ago my truck ran great until I found out it was burning oil in the #1 cylinder (oil fouled spark plug). Then, it had that bad idle (kind of what I have right now). I feel the idle has gotten worse because several months ago I found out that the #8 cylinder had an oil fouled spark plug. But both spark plugs have extensions of them. Now, while no wet oil can be seen on either of the spark plugs (because of the extensions), carbon builds on them and you can see a little bit of wet oil on the top of the extensions when pulled out of the engine. 

I also thought it was possible for the valve seals to be bad and the oil could be dripping down the engine onto the spark plugs when the engine was turned off.

I also want to say that -- when I was testing the EGR valve with the vacuum gauge in the first few posts of this thread -- my engine ran 100% in idle and throttling up in idle (in park). No hesitation. Almost like if EGR valves did not exist, then my engine would be running fine.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Seafoam.........., thats what I used to use a long time ago. for the life of me I couldn't remember what that stuff was called. I've been using gumout on mt Blazer for the past 8 years, and so far the only code its ever poped is when the map sensor literally broke the vacuum line nipple off.

Kevin. If Seafoam doesn't do it, you will have to manually yank the EGR valve off, as for your truck burning on 1 and 8, unless its fouling plugs and causing cops to notice you, you might be able to run a Chevy like that for years, my old 76 burnt on every cylinder, and all I did at 300000, is use straight 30 in it until old age took the engine.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin

Troubleshooting vacuum leaks doesn't mean that you have to " hear it " to determine if the engine is losing vacuum. There are mechanical factors that play a pivotal role in how the engine performs. As you are well aware, vacuum is created when the piston is on the downward stroke when the intake valve opens. Worn out valve guides, seats, seals, piston rings, have an enormous impact on how the engine runs. To check for leaks, you do not need to hook up a vacuum pump and hook it up to every hose, all you need is a vacuum gage and the knowledge of what the readings on the gage represent. 

Here is a link showing how to troubleshoot an engine with just a vacuum gage readings: http://www.earlycuda.org/tech/vacuum2.htm


The brake booster has a diaphragm inside, if that diaphragm is damaged in any way then the answer is yes, the booster can cause the vacuum leak( extremely rare). But, for the brake pedal issue, without having the booster the pedal should feel very stiff because it will not have the power assist. The brake pedal will go down only if there is air inside the system or if the brake system leaks fluid. A simple method to check the master cylinder for air or leaks is to disconnect the booster, then take vice grips and clamp off all the flex hoses. This will isolate the master cylinder, apply the brakes normally and see if the pedal is at it's peak height. The pedal should feel like a rock under your foot. If the pedal drops slowly no matter how small a distance, then the master is leaking internally or there is air inside it. 

As for the oil burning problem, if the valve seals are damaged or if the rubber has hardened, then the engine oil would be getting into the cylinder. Bad valve seals is not the only thing that will make the engine burn oil. Improper valve seating or if the valve guides are worn will also cause engine oil to seep into the cylinder. 

Kevin, the EGR should only open at wide open throttle. If you push the EGR diagram up with your fingers, the engine will stall ( at idle). If the engine doesn't stall then EGR is leaking from somewhere, the diaphragm is damaged, or the intake manifold has a hairline crack in it.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hey octaneman the booster diaphragm isn't as rare as people think, at least around here anyway, every time i remove a master cylinder and theres fluid on the back side, I end up replacing a booster too, not just on mine, buy everybody I do brakes for. I don't know if its just this area, or what, but master cylinders leak all the time.

Kevin.. Black smoke usually = bad fuel air mix, could be vacuum, could be air, 
Blue = oil, we all know that one
but light Grey? last time i saw that it was trans fluid dripping on the exhaust.

But yes it is possible for valve seals to go ban and oil drip into the cylinders, my Blazer does that every now and then


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## SABL

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Light grey smoke means water in cylinder... head gasket, cracked head or block. Not always but generally the cause.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



SABL said:


> Light grey smoke means water in cylinder... head gasket, cracked head or block. Not always but generally the cause.


Unless you drive a VW bettle with the air cooled engine, then white smoke means Cheech and Chong are in your car 

SABL I always found water leaks the old fashioned way on Chevy's, pull the dipstick and look for the milkshake.


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## SABL

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

@wolfen:lol:

Good point with the VW.... I had a '71 Super Beetle.

Seen the milkshake effect with a relative's boat.....:4-thatsba:4-thatsba

Lack of winterizing destroyed a 4.3 Merc.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

SABL heres another good one for ya, on the old vehicles that have a mechanical fuel pump bolted to the Engine with a cam driven shaft, if the oil looks like a milkshake and smells like gas, the diagram in the back of the fuel pump is shot. But most of those are either gone now of have antique plates on em.
Seriously though I have never seen white smoke from an engine because of water, I always thought that would cause steam and a massive amount of water to pour out the tail pipe, but if trans fluid is dripping onto the exhaust its lt Grey to white (ref. my old 73 Pontiac wagon with the trans cooling lines dripping on the exhaust "Y" pipe)


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## SABL

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hi wolfen,

We've kinda strayed from the OP.....OOPS!!

A small breach between the head and block can and will produce white smoke from the exhaust if the seal is broken between waterjacket and cylinder. If your lucky, the water vaporizes and prodeces white smoke.... if not so lucky, water accumulates in the cylinder and buckles the connecting rod....OUCH.

Not being snippy, but your Poncho (Pontiac) was producing white smoke from under the vehicle and not from the exhaust, but by the exhaust. Another source of white smoke from under the vehicle (best noted at night with the headlights on) was "blow-by" before the advent of the PVC system when crankcase gasses were vented into the open air. 

I remember the mechanical fuel pumps rather well..... with a drive-rod actuated by a 'concentric' on the front of the camshaft. Had to get the rod in the right position or fight the pump to get it installed..... just one of life's little lessons. Went electric in '69 with my 283 ('57 Chevy 210) and had to get used to the "buzz" when I turned the ignition on. In a way I kinda miss the old mechanical fuel pumps..... at least you didn't have to drop the tank to replace them!!


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Your right, it wasn't the exhaust causing it, it was dripping from the trans cooling lone onto the exhaust pipe right at the little mount block and burning off.

I've never had one get enough water to lock it up, but I did have an old Nova once that knocked real bad until you gave it gas, once time I gave it just a little too much gas and teh rod was perfectly visible through the hole in the block LOL. Sold the Nova for 500


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## Wrench97

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Water(Antifreeze) and oil burnt in different cylinders can cause the smoke to look gray.
Also it could be a rich mixture(black) combined with oil(Blue) to make it look gray.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Wolfen1086,

When you drove your 76 Chevy with straight 30 oil and your truck burned oil in every cylinder, did you notice any idling/hesitation problems? Was your idle low like it wanted to die? When you accelerated at a complete stop, did your engine have the nice "Chevy rumble" sound or did it have the "hesitation sound?" I hope you know what I mean. It's hard to explain here.

I'm trying to see if my idling/hesitation is from something electrical/emissions or if my problem is due to oil burning in my #1 and #8 cylinders.

Also, I just want to say -- about a month ago, I saw a lot of blue smoke coming out of the exhaust. I put in 20W 50 oil. Truck didn't smoke again. Then, after about a week or two of putting that oil in, I went down to 10W 40 and then 10W 30 (since it is getting colder out). I haven't seen blue smoke since I put the heavier oil in my truck.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

SABL and wrench97,

Thanks for replying to this thread.

I thought about the light gray smoke being the headgasket, but my truck has never overheated, my coolant in the resevoir has always been checked and at the reasonable level, and my oil is black as ever (no milkshake).


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

I have an update.

It has been colder during the mornings and nights here. When I start my truck up after it sat overnight, the truck runs pretty good. No hesitation and when I reach a stop sign, the truck idles perfectly (normal idle). However, when the engine heats up and the temperature gauge hits a little under operating temperature, the truck idles low again.

Cold = engine running good. Hot = engine not running like it should be.

Could this be something electrical heating up and dying out (like a pickup coil) or could this be the piston rings or some other worn rubber that is heating up and stretching creating problems within the engine (low compression, etc.)?

Also, I cleared the code 32 (in my earlier posts) and I drove it the past three weeks which is why I haven't posted in a while. My engine service soon light did NOT come back on; however, I went outside about 20 minutes ago and I decided to hook up my code scanner just for the heck of it. 

I received a code 34. That was the only code it showed. Code 34 is MAP sensor (voltage low at ignition ON). That was the definition in my book for that code. In my earlier posts, I checked the voltage and it seemed ok, but I'm going to re-check it.

I still have to do the vacuum check, I'm going to redo the MAP sensor test, I'm going to test the EGR valve again (try to push it up and see if engines stalls out), and I'm going to do a compression check.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin

Heat does affect the engine's electrical components by changing it's internal resistance, the computer will take those voltage values and make adjustments accordingly. Loose ground wires affect the signals going to the computer instantly due to bad surface contact, this creates an intermittent open circuit. Those types of electrical faults are more difficult to isolate but not impossible. By removing and cleaning all ground contacts like the battery negative cable you eliminate that possibility. 

Please post back the results of your tests.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



Kevin350 said:


> Wolfen1086,
> 
> When you drove your 76 Chevy with straight 30 oil and your truck burned oil in every cylinder, did you notice any idling/hesitation problems? Was your idle low like it wanted to die? When you accelerated at a complete stop, did your engine have the nice "Chevy rumble" sound or did it have the "hesitation sound?" I hope you know what I mean. It's hard to explain here.
> 
> I'm trying to see if my idling/hesitation is from something electrical/emissions or if my problem is due to oil burning in my #1 and #8 cylinders.
> 
> Also, I just want to say -- about a month ago, I saw a lot of blue smoke coming out of the exhaust. I put in 20W 50 oil. Truck didn't smoke again. Then, after about a week or two of putting that oil in, I went down to 10W 40 and then 10W 30 (since it is getting colder out). I haven't seen blue smoke since I put the heavier oil in my truck.


Nope nothing except a plume of blue smoke out of the tail pipe, but also you gotta remember a 76 had 0 emitions on it also, so there was nothing to stop all 8 from firring if they all had fuel air and fire. I did hesitate one time when I had a 250 straight 6 in it, but that was a cracked in the irrigated head


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hello there,

It's been awhile for this thread, but I really need to revive it. I'm glad it's not closed for inactivity. I appreciate that.

In the previous posts, I said I was going to do the different tests for the vacuum test, EGR valve, and the MAP sensor, but I honestly got lazy. And, since my truck was running at the time, I put those tests off. I just drove it like it was.

It's been about a year since I replaced the ignition control module and the ignition coil in the truck that solved the problem last year. But the same problem came back. And, today, my truck stalled out while coasting down a road once again. I took off the air cleaner and looked inside and while the truck was cranking over there was no fuel. I tried to listen to the fuel pump to see if it would turn on, but there was no sound.

*Here's the best part:* It's the same problem from last year. Last year we thought it was the fuel pump (I didn't see any gas going into the TBI/carb), so we changed the fuel pump. A new fuel pump did not solve the problem as the truck was still stalling out while in idle (in park and in gear). No sputter, no backfire...just shut down smoothly. And, when it shut down, it just kept cranking over and didn't start. But we changed the ignition control module and the ignition coil...and it solved the problem. How would changing those two parts help the truck get fuel?

Now, it stalled out on me just like last year. Same symptoms, same problem. The ignition control module and ignition coil fix lasted for exactly one year.

I know if I change the module and ignition coil, it would most likely solve the problem once again. But what is shorting them out? A faulty distributor? The only part that I can think of going bad regarding to fuel is the fuel pump relay which has never been changed. The relay is 20 years old.

I finally bought a digital multimeter, so I can test anything electrical if anyone has an idea on what the problem could be. It's not a Fluke, but it has pretty good features on it. I also bought a new digital camera to take pictures if anyone needs them.


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## Wrench97

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Check the fuel pump relay first.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Wrench97,

Thank you for the reply! You were right. I checked the relay and the fuel pump relay fuse was blown out. I replaced it with another #20 fuse, truck took a few cranks, but it started up.

I'm still wondering if something is shorting out the fuse though as I have a feeling this will happen again.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hi Kevin 

Good to hear from you again, it's a good thing you picked up the the meter because you're going to perform a few tests to solve this problem. In you're post you mentioned that there was no fuel going to the injectors while you were cranking the engine. There are several possibilities that can cause the problem, but it's not ignition components so you'll need to break out the meter and start troubleshooting. First thing is you will need is a little background info on how the injection system works. Injectors work with a pulse that is generated by the computer, the time the injectors turn on/off its called pulse width which is measured in milliseconds. A node light is usually inserted into the injector plug to see if the injector pulse is present, once inserted you turn the ignition on and while cranking the engine the node light will begin to flash on/off very quickly, this will tell you whether the pulse is there, intermittent, or not at all. Injectors are grounded to the computer, and the computer is grounded to the chassis, so if no pulse is present the injectors won't open so the problem is either a bad ground to the chassis or wires to the computer. To test the injector wires, you will need to do a continuity test from the computer harness pins to the injector plug pins one at time, but you will need a schematic to do this test. The meter will tell you if the circuit is open or closed, when the circuit is closed you will get a reading on the meter, if its open you will get no reading at all but it won't be 0.00 so don't get fooled. 

Injectors can also get blocked, leak, or it's internal resistance gets so high that it shuts off because of excessive heat build up. Injectors have to be tested both hot and cold to make an accurate assessment of its performance. To do this, take out your meter and set it to ohms scale, then remove the injector plug and connect your leads to the pins on the injector. With the engine cold, key off engine off take your readings and then do the same test with engine hot. The readings should be between in the vicinity of .017 ohms (check your spec sheet) if your readings go below .012 then the injector is gone. 

When injectors go bad, all of them have one thing in common....A dirty fuel filter. That is why it is very important to have your filter changed at when you do your tune ups, or if you suspect the engine is running sluggish. Injectors also have a spray pattern which is cone shaped when the injector is working properly. The best way to see the pattern is by hooking up a timing light and then point the light to the injector, if there is any defect in the pattern it will be clearly visible.

The next test you will need to do is a continuity test from the fuel pump harness connector pins to the fuel pump relay. Again you will need a schematic to find out which are the correct pin outs. Look for any loose, broken, or corroded wires, hook up your meter to the relay pin junctions set your meter on ohms scale ( continuity test) do a wiggle test on the harness by shaking it and see if the meter readings jump. If they do then you got a bad connection that needs attention. 


post back your findings.


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## Wrench97

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

The Fuel pump relay fuse, I believe also powers the fuel pump itself, ? If the wiring to the pump looks intact along the inside of the frame rail then you may have a failing pump, usually when the tank gets low on fuel(1/4 tank & lower) the pump motor runs warmer because the fuel is not covering it to cool it, how much fuel was in the tank when it stopped?

If you remove the fuse and probe the relay socket there should not be power at any terminals if there is then forget what I said above that fuse only powers the relay and not the pump.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hey Octaneman,

Nice to hear from you again too. I will definitely do those tests as I'm afraid the truck will stall out on me. And, of course, it stalls out in the wrong places.

I just want to post this (just in case) to give an update and see if you have additional information about this. It may change the situation, but it may not. 

*Here's an update:* I posted my message earlier in the morning saying the new fuel pump relay fuse fixed the problem. After I posted that, I went to go drive the truck. It went to about operating temperature (probably about a 5-8 minute drive) when the truck stalled out again. I looked under the hood and the second fuel pump relay fuse shorted out. Luckily I have 3 left...placed another new one in and off I went. My father suggested to buy a new fuel pump relay. It was only $16 and I placed the new one on. I should be testing them before throwing the parts at it. But I gambled with it.

Took a 20 minute drive...drove it hard...stopped hard...let it coast up and down hills. It was a lot longer drive than the drive I took this morning. But with the new fuel pump relay in it and the longer drive...it *did not* stall out on me.



wrench97 said:


> The Fuel pump relay fuse, I believe also powers the fuel pump itself, ? If the wiring to the pump looks intact along the inside of the frame rail then you may have a failing pump, usually when the tank gets low on fuel(1/4 tank & lower) the pump motor runs warmer because the fuel is not covering it to cool it, how much fuel was in the tank when it stopped?
> 
> If you remove the fuse and probe the relay socket there should not be power at any terminals if there is then forget what I said above that fuse only powers the relay and not the pump.


Wrench97,

The fuel tank is a little more than 1/4 full when the truck stalled yesterday and today. But I just replaced the fuel pump relay today, took it on a longer drive, it easily hit operating temperature and the truck did not stall on a 1/4 tank of gas.


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## Wrench97

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

It would be unusual for the relay to be shorted, bad contacts inside could lower the voltage raising the amperage draw maybe but I kind of doubt it.


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## kjms1

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

and if the fuse was 20 years old it may of just got tired out and let go ... nothing last forever
a fuse blowing one times does not mean there is a prob now twice in a short amount of time will let you know there is a problem in that circuit


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

From your last post the truck seems to be running OK, but just in case you have doubts about it here's a little quick test you can do in your spare time. It is called a "wiggle test" it is done when wires are suspect, with key on engine running grab hold of the wiring harness next to fuse/relay box and shake it do the same thing on the rest of the harness. Note how the engine reacts, if it appears that it may stall then inspect the wires to the fuel pump relay and the fuse under the box for looseness or bad grounds.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hey Octaneman,

I had to put off the few tests for a while because the supply and return fuel lines were rusted near the fuel filter and it was slowly leaking fuel. Replaced both fuel lines along with a new fuel filter. I noticed it while replacing the rear brake line from the master cylinder all the way down to the differential. 

1. I pointed the timing light towards the injectors (throttle body injection). The left injector seems to be spraying fuel perfectly while the right injector is giving fuel, but it looks like it's dripping a little. It could be dirty.

2. When I was testing the injectors (not the wires) with the multimeter, I had my meter on "Auto range." With the engine hot, both injectors read 1.7. If I tried to manually change the decimal place on the meter, the numbers would just stay on .001 or .000. My meter would only pick up the numbers in tenths and not thousandths (.017). I don't know why, but is that a problem? 

3. I also tried wiggling the wires of the fuel relay wiring harness and nothing changed. There was no change in how the engine reacts.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hey Kevin 


Looks to me that you nailed the injector problem, a leaky injector will mess up the spray pattern and will give a false positive readings on the meter. A false positive means that the readings on the injector will look OK, but since the cone pattern has been obstructed that the meter readings will go up and down preventing it from getting a solid fix on it's resistance. When that happens only a visual inspection of the pattern will decide whether or not to replace the injector. 


The decimal point on the meter indicates the range values. Ohms (limit range 1-999 Kilo/ohms (limit range 1000-9999) Mega/Ohms (limit range 10000-1,000,000) For example: If you have a reading of 4.7K Ohms and you toggle the decimal point on the Ohms scale on the meter. The meter will read it as 0.00 because the resistance value has exceeded the limit value of the lowest scale. (Limit values are set by the manufacturer of the meter so check your documentation how the scale works ). When you toggle the switch to the next higher scale (kilo/ohm range) the meter will pick it up ,the meter will read 4.6 kilo/Ohms because it is within the scale limit resistance value. In the highest scale your meter may still read it because it is within limits, but it will read it as 0.0005 because it is too low on the Meg scale but high enough to pick it up. 
I hope this clears up things on how to read the meter. 



If the engine wires passed the wiggle test then you can rule out bad connections. Now you have to focus on the mechanical aspects of the engine; Valves, timing chain, vacuum leaks etc....


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Addendum:

Kevin 
I forgot to mention that certain DVOM's and multimeters have different ways of displaying readings. The most common reading you will see on many meters is the display of OL. The interpretation of OL is NOT Over Load as most people believe. The real meaning of OL is Outer Limits ( infinite resistance). Outer Limit means that the resistance of the object you are testing has passed the the meter's capacity and sees it as an open circuit. That is why it is extremely important to ALWAYS have the spec sheet with you at all times when testing ANY object with your meter.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

Thank you for all the information on the meter. 

After I recently did new fuel lines and fuel filter from the TBI to the gas tank (because they were rotten), I noticed my truck is pinging more than it used to. I've been putting in 93 octane for a couple years now, but I really have noticed it pinging more now than before I did the fuel lines. 

When I find the time next week, I'm going to check the timing, vacuum, the EGR valve again,etc. I'll even check the fuel pressure and compression since I have a new fuel pressure and compression tester.

Since I've seen white crust under the distributor cap, I'm also going to try and test the whole distributor with the meter and see what's going on with that.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin 

Engine pinging can be caused by several possibilities:

1) Carbon build up inside the engine
2) Low octane
3) Ignition timing too far advanced 
4) Blocked catalytic converter 
5) Overheating

Start by checking the most common things like timing marks re-set it if its off. If your truck is equipped with a clutch fan make sure its spinning when the engine reaches full temperature. A while ago you had a carbon issue with the engine, was it ever resolved ? This does not appear to be a random pinging problem, the timing of it is too coincidental.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Octaneman,

Unfortunately, I still have the carbon issue. And that's not going away until I find out whether the valve seals/guides are worn or the piston rings are shot because the truck burns oil.

Checking the timing is long overdue. I have to do that.

The catalytic converter most likely needs to be replaced. It's probably bad from wear and tear alone. But, since I have the carbon issue, the converter is most likely carboned up too which kills the insides of it.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Kevin 


Here's a link explaining valves that leak oil and what the pros and cons of the various methods used to correct the issue. 


http://www.aa1car.com/library/ar696.htm


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

^^^^^^^what he said, also there is a product out there called umbrella valve seals, they stop oil from going through a worn valve seal and collecting in the cylinder. Some people will tell you that you can install them to correct the angle on a worn valve seal, DO NOT believe any of them, that seal is ONLY for worn "O" ring at the top of the valve guide


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



octaneman said:


> Kevin
> 
> Engine pinging can be caused by several possibilities:
> 
> 1) Carbon build up inside the engine
> 2) Low octane
> 3) Ignition timing too far advanced
> 4) Blocked catalytic converter
> 5) Overheating
> 
> Start by checking the most common things like timing marks re-set it if its off. If your truck is equipped with a clutch fan make sure its spinning when the engine reaches full temperature. A while ago you had a carbon issue with the engine, was it ever resolved ? This does not appear to be a random pinging problem, the timing of it is too coincidental.


Hey Octaneman,

How are you?

The truck is still pinging. Two weeks ago, I put an almost full tank of 87 octane in because I wanted to be cheap and not pay for 93 octane. Big mistake and I knew I shouldn't of done it. Now, the truck is pinging and backfiring. It doesn't backfire in first gear. It backfires in second and third gear, especially on the highway and going uphill. I filled the gas tank up from a half a tank with 93 octane about three times now and the truck hasn't cleared up yet.

No overheating.

Carbon issue is still there, but it never backfired until I put the 87 octane in.

I haven't checked the catalytic converter yet.

I had to clean white crust from the terminals inside the distributor cap. 

I checked the timing today. My settings are supposed to be at 0 before top dead center. Before top dead center, it was at 4. So I set it correctly to 0 before TDC.

I noticed my truck only backfires at highway speeds, under a heavy load, or going uphill, so I checked the EGR valve with a vacuum pump. The solenoid holds vacuum, so that's ok. When I pumped the vacuum gauge into the EGR's vacuum hose, the engine idle speed slightly went down, but it seemed fine. The truck did not run really rough and it did not stall. In my book, that means the EGR valve is not working properly. The EGR valve is from NAPA.

Should I change the EGR valve again and go to a GMC/Chevy dealership to get an OEM EGR valve? Or is there something else that I could check? Is there something that I could buy for the fuel to boost the octane and see if it clears the pinging/backfiring up? 

I feel that it's something so simple to fix. I really don't want to junk the truck over this.

-----------------------------

*Wolfen1086*: As for the valve job, I'm waiting for more warmer weather to check the seals/valves/guides.


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## Wrench97

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

It may mean the ERG ports are blocked with carbon deposits, remove the valve and see if it opens and closes, the intake manifold and exhaust ports in the center of the heads can become blocked and not provide the exhaust gas to the valve.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hi Kevin

Good to hear from you again. 

Wrench's diagnostics is spot on, the intake ports to which the EGR is connected to are probably blocked. Disconnect the vacuum hose to the EGR, start the engine normally then block the vacuum hose with your thumb. Snap the throttle to about 1200 RPM and feel if there is any vacuum coming through, if not run another treatment of seafoam into the intake to clear out the carbon. Installing OEM parts are always preferable because of the quality but are pricier to jobber parts, if your budget supports getting an OEM one then go for it, but be sure the ports are clear before spending the cash on a brand new EGR. 


For the fuel you can pick up some octane booster additive, but be aware that there will be higher emissions and the engine will run hotter with higher octane. Not all engine's can run higher octane because of the casting process in materials, pistons can shatter because of the higher percentage in fuel burn. The engine's RPM has to be monitored and the timing has to be adjusted slightly to prevent dieseling (engine spinning backwards when you shut off ignition) or serious damage may occur. Your engine runs on regular gas and the main problem is carbon build up, if its really caked on the only option you got is fishing it out by hand. If we can help you give a knock-out blow to that issue then the octane boost won't be necessary. 


Question: Is your truck equipped with an anti-pollution pump that runs off the power steering ?


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## Midnight Tech

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Just curious - how much ethanol are they putting in the fuel there, Kevin? I've noticed a lot of vehicles will ping with the enriched gas.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



wrench97 said:


> It may mean the ERG ports are blocked with carbon deposits, remove the valve and see if it opens and closes, the intake manifold and exhaust ports in the center of the heads can become blocked and not provide the exhaust gas to the valve.


Wrench97, thank you for the advice! I'll check it out.



octaneman said:


> Hi Kevin
> 
> Good to hear from you again.
> 
> Wrench's diagnostics is spot on, the intake ports to which the EGR is connected to are probably blocked. Disconnect the vacuum hose to the EGR, start the engine normally then block the vacuum hose with your thumb. Snap the throttle to about 1200 RPM and feel if there is any vacuum coming through, if not run another treatment of seafoam into the intake to clear out the carbon. Installing OEM parts are always preferable because of the quality but are pricier to jobber parts, if your budget supports getting an OEM one then go for it, but be sure the ports are clear before spending the cash on a brand new EGR.
> 
> Question: Is your truck equipped with an anti-pollution pump that runs off the power steering ?


Octaneman, thank you for the advice! 

My truck does not have a smog pump.

Will the Seafoam hurt my truck? I know it eats the carbon away, but since my truck is old, will the Seafoam eat away the insides of my engine, especially all of the rubber gaskets?



Midnight Tech said:


> Just curious - how much ethanol are they putting in the fuel there, Kevin? I've noticed a lot of vehicles will ping with the enriched gas.


Midnight Tech, I'm pretty sure it's 10% ethanol.


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Seafoam is perfectly safe but if you got reservations about it just run it through the intake and not the crankcase.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*



Midnight Tech said:


> Just curious - how much ethanol are they putting in the fuel there, Kevin? I've noticed a lot of vehicles will ping with the enriched gas.


"Supposedly" its E-10 everywhere in the U.S. Except for certain rural areas where the farming equipment requires pure gasoline. But if I understand the email from a friend of mine correctly its illegal to put pure gas in a car now, we are being forced to use E-10.
The only good thing about alcohol is its s good antiseptic and you can drink it, Its NOT meant to mix with gasoline.
Just wait until later this year when the administration signs the E-15 bill, yall think we have problems now..............


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hello Octaneman,

How are you?

Unfortunately, I have another problem with my truck. Today, my fuel pump fuse (next to the fuel pump relay) blew out while the truck was coasting down the street (I was not giving the truck gas at the time).

In September 2010, it did the same thing. I replaced the fuel pump relay and the fuse never blew out until today.

Do you know what to test or look at to fix this?


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## octaneman

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hi Kevin,


Glad you kept in touch, 

Fuse blow outs on fuel circuits are generally caused by fuel pumps shorting out or bad grounds. On trucks like yours that have done the mileage, the realistic thing to do is go to where the sending unit is and inspect the wiring harness. Wires that have been exposed to the elements for years rot out, sometimes it is not visible with the naked eye so doing a resistance and continuity test on key points helps you get a better picture on the wire's internal state. Go to the rear of the truck where the harness is hooked up, look for corrosion and loose ground wires from the sending unit. Do a wiggle test on the molex connector pin wires and make sure they're clean and make good contact especially the ground side. have an assistant with you, hook up your meter to the molex connector pins relay side and take voltage reading, pay close attention to the three second delay relay activation (check engine light on/off) when you turn the key on (engine off). Listen to the pump, this will give you an overall picture of the sound the pump makes and the wire's state from front to back.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Thanks, octaneman. I appreciate it.

The truck is running and the fuel pump fuse hasn't blown out yet. So it seems that buying a new fuel pump relay is a temporary fix for 6-7 months.

I'll still take a look at the things you said in your post.


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## Kevin350

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Hello Octaneman,

It's been a while, but I came back to post that the problem has been solved with this truck. 

The truck recently blew out another ignition control module while pinging (with 93 octane) and backfiring under a load.

I took a look at the distributor pickup coil. The pickup coil wires were corroded and shot. It's possible they could have been arcing and blowing out the electrical system. Also, with a faulty distributor pickup coil, it was most likely screwing up the timing of my truck. A year ago I set the timing to 0 degrees. I recently checked the timing three weeks ago while replacing the faulty pickup coil and the timing was off 4-6 degrees causing the pinging and backfiring.

I replaced the distributor pickup coil three weeks ago and haven't experienced any backfiring or heard a single ping while driving it in stop and go traffic or on the highway under a load (accelerating uphill).

Thank you for all the advice that you have given me. I appreciate it a lot.


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## wolfen1086

*Re: 1990 Chevrolet C1500 -- Service Engine Light Turns On At Highway Speeds!*

Glad you got it running right.


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## octaneman

Anytime Kevin, 

Glad it worked out.


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