# chevy 1500 surging in overdrive



## redchevyz71 (Dec 17, 2010)

I have a 94 chevy 1500 5.7l throttle body fuel injection. The problem is that when I'm in overdrive or low gear on the highway the engine rpm surges up and down usually between 1500 and 1600 rpm while in overdrive. After about 10 min of this the service engine light comes on and throws an o2 sensor. Any ideas? I've replaced the injectors, fuel pressure regulator, pcv valve, idle air control, tps sensor, o2 sensor and a list of more stuff... I also have a tbi spacer idk if that could be doing it?


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## octaneman (May 13, 2009)

Is there vacuum going to the MAP sensor ?


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## redchevyz71 (Dec 17, 2010)

Yea the vacum seems fine. But I did make a mistake, I got my friends scanner last night after I got the check engine light to come on and the codes were dtc 32 and 42. 32 is egr and 42 is timing spark? I think I need to replace the egr but would a bad egr cause the other code to come up?


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## octaneman (May 13, 2009)

Yes it would , If the EGR is leaking it would throw off the timing.


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## redchevyz71 (Dec 17, 2010)

Alright thanks so much I'm pretty sure its just a bad egr valve considering its the original with 135000 miles. I post back when I replace it and see if there is any difference


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## redchevyz71 (Dec 17, 2010)

I replaced the egr valve, it still does it but now the codes are 32 33 and 42. 33 is the manifold absolute pressure high which means the vaccum is low. What would cause this?


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## octaneman (May 13, 2009)

Vacuum low pressure is when there is a restriction or complete blockage in the amount of vacuum going to MAP the sensor. The problem you have to contend with is the code 42 which the computer has detected no spark advance. The problem can come from the ignition module or the computer.


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## redchevyz71 (Dec 17, 2010)

Ok so is the 42 code setting off the other 2 codes? And where would I begin to solve this? I just timed the truck the other week and there is a timing connector wire you need to remove before timing. After I timed it I plugged the wire back in. Should I have disconnected the battery after that?


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## octaneman (May 13, 2009)

Its very possible, you should erase the codes and see if the CEL comes on again.


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## redchevyz71 (Dec 17, 2010)

Well tryed resetting it and that didn't do anything. Is the 42 code causing the other two codes or vise versa? And if so where should I start to try and fix it?


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## octaneman (May 13, 2009)

Start by taking your DVOM and test open to ground the advance wire from the ignition module to the computer. If the wire tests OK then the ignition module has to be tested for the pulse signal to be present and to do that you need an oscilloscope or labscope. The old fashioned method is replacing the ignition module first, if it doesn't solve the problem the computer gets ditched as well.


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## redchevyz71 (Dec 17, 2010)

Could it just be a bad map sensor? I have a chilton book and it says that with the key on the sensor should read 5 volts. Mine read about 3.2 volts. Wouldn't that mess with the ignition timing?


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## octaneman (May 13, 2009)

Yes it would, if no vacuum goes to the sensor or the diaphragm is messed up in some way it can set off timing codes because the computer adjusts air/fuel by what the sensor tells it. By probing the green wire to positive on the meter and the black wire to body ground with KOEO the sensor should read 5v. The MAP sensor readings should begin to drop when vacuum is applied, for every 5Hg of vacuum the sensor should drop 1 volt. The sensor should also be able to hold vacuum if it doesn't its gone, remember sensor voltage changes as engine load changes.


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## redchevyz71 (Dec 17, 2010)

Ok thanks I'm going to test that and the egr solonoid. I'm not sure if I did not reset the computer correctly but I left my battery disconnected over night and now the only code is 32 which is the exhaust gas recirculation. Considering it is a brand new egr valve, the only other thing I can think of is the egr solonoid


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## octaneman (May 13, 2009)

Is your EGR vacuum actuated or electronic ?


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## redchevyz71 (Dec 17, 2010)

Vaccum


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## redchevyz71 (Dec 17, 2010)

Atleast I think so? Its got one vaccum line and that's it


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## octaneman (May 13, 2009)

Trace the vacuum line, does it go to a solenoid or to an intake port ? If you have a hard time following the vacuum line look at your sticker under the hood that has the vacuum diagram for your engine is there an EGR solenoid ?


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## redchevyz71 (Dec 17, 2010)

Okay last night I went over to my mechanics house and we were checking some stuff out. Yes there is a egr solonoid. We first took a vaccum line straight from the throttle body to the egr valve will little difference in idle. He told me that it should stall or almost when the the solonoid is bypassed. He figured the ports in the manifold where the egr valve bolts up must be clogged with carbon. Next we tested the solonoid by starting the truck then taking the two prong wire harness and grounded one and running another wire to the positive with no difference. We came to the conclusion that the solonoid is shot because when shorted out it does not do anything so I will be replacing that tonight and posting back my results. Thanks for your help I can normally figure most of this stuff out but I needed someone on hand to help as well.


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## redchevyz71 (Dec 17, 2010)

Well I replaced the solonoid and it is still doing it...


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## octaneman (May 13, 2009)

The EGR solenoid has a 5v reference pulse that comes from the computer, the vacuum solenoid opens at a specific value of approx 5Hg's ( check your engine specs) which will allow vacuum to flow to the EGR. What you will have to do now is hook up a vacuum gage to see how much vacuum goes to the EGR. If the intake ports are blocked by carbon deposits the solenoid wont function, you will have to probe the port with a piece of coat hanger or some other object to brake apart any deposits trapped inside. The EGR solenoid activates at around 2000 RPM, so by hooking up your gage to the EGR vacuum hose and then throttling up to reach the designated engine RPM's, place your fingers under the diaphragm and feel if the EGR opens. If it doesn't open then the problem is carbon blockage, that is as long as there is no vacuum leak coming from any of any of the vacuum hoses. A quick test is to run the engine at idle then push the diaphragm up with your fingers and the engine should stall, if it doesn't stall and it just stumbles then the passages are blocked. 


Another method in testing the solenoid is with a vacuum pump, but to do this you will need to set your engine computer in service mode ( engine off) by connecting pins 5 and 6 of the ALDL connector. With the computer in diagnostic mode leave only the vacuum hose connection from the EGR to the solenoid. Disconnect the hose from the EGR valve and hook up a vacuum gage, then hook up your vacuum pump to the solenoid side of the intake. Apply vacuum and whatever reading your pump says your gage reading should be identical, if not then the hoses leak which will not allow the EGR valve to activate. 

The 5v reference pulse is equally important if by some reason it isn't there on the molex connector, then there is an open to ground either from the wires or the computer. It does not happen often in fact its rare but it does happen.


post back your findings.


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## redchevyz71 (Dec 17, 2010)

Alright let's start with the possible blocked passages. I took the egr valve off and started the engine and used a screwdriver and coathanger to try and break apart the carbon. I started the engine so that the carbon would be sucked through the exhaust. With the egr off of the manifold should the engine stall? Mine did not and I scooped out a lot of carbon out of the manifold with no change, the code still comes up. I'm getting really frustrated with this and I'm not sure what to do. I'm not 100 percent sure how the egr works but would having aftermarket headers cause that? Also my header gaskets need to be replaced and it is leaking would that do anything? Sorry for all of the questions i just don't really have a clue where to start with this.


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## octaneman (May 13, 2009)

Yes the engine should have stalled and from your observation the EGR passageway is blocked. The function of the EGR is simple when the valve opens it redirects the exhaust gasses back into the intake, if there is a pipe threaded on to it as some models have it also goes to the catalytic converter. Since the exhaust gas is inert it can't be burned, so it mixes with the fresh intake air and reduces NOx ( nitrogen oxides) and also brings down the cylinder temperature to prevent the engine from pinging. 


Carbon is made by blowby gasses that pick up particles and then they begin to solidify when colder air is thrown into the mix. Leaky headers will no doubtedly accelerate the carbon build up issue because cold air gets sucked in from the gaskets. Worse case scenario is that the throttle body has to be removed and the carbon flushed out manually.


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## redchevyz71 (Dec 17, 2010)

Okay I kind of understand it now. So what is the best way to get the carbon out of the passage ways? I can try using the clothes hanger again but I wasn't having much luck with that


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## octaneman (May 13, 2009)

First thing you do is remove the EGR, then with the throttle body out you feed the coat hanger through the port on the intake plenum. The port is a small hole through the plenum that goes all the way to where the EGR sits. That is the passageway you have to clear out. The easiest way is to remove the throttle body and clear it out with carb cleaner while poking it with the hanger, because I fear that in your particular case the carbon has set like concrete and you may not have a choice in the matter. If you do go that route don't forget the torque sequences on the throttle body when you install a new throttle body and EGR gasket.


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## redchevyz71 (Dec 17, 2010)

Well I finally got the egr ports cleared out. I had my mechanic work on it for about 3 hours before he got it done... but of course the ses light still comes on. I pulled the headers off because I need to fix the leak. The flanges are warped (which clearly caused the leak) so I'm getting those machined down. If the exhaust leaks are fixed will this stop my engine from surging and the light from coming on when I'm just giving a little throttle in od??


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## octaneman (May 13, 2009)

Fixing the exhaust leaks and clearing out the EGR ports will definitely go a long way in the final resolution to the problem. Just make sure that there are no vacuum leaks whatsoever in the final assembly and that the timing is dead on. 


Question: Did you test the 5v reference to the EGR solenoid is it there ?


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## redchevyz71 (Dec 17, 2010)

Yea I have checked and replaced almost all of the vaccum lines and the timing is fine. I did some searching around and found that if a ground wire is disconnected it will throw codes like that. Sure enough I found a ground wire on the passenger side of the block that was burnt through by the header so I replaced that and now I'm just waiting for the headers to get machined down. I'm crossing my fingers that this will solve my problem


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## redchevyz71 (Dec 17, 2010)

Replaced the ground wire and got the headers sealed up pretty good but once again the ses light comes on after little while of the engine surging at highway speeds... any other ideas?


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## octaneman (May 13, 2009)

Is the CEL a code 32 ?


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## redchevyz71 (Dec 17, 2010)

Yea I believe so. I don't have my friends scanner anymore but I don't see what else it could be considering its doing the same thing. Would a bad tranny cause something like this to happen? because mine is on its way out...


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## octaneman (May 13, 2009)

Second guessing codes will waste resources, what you need to do is erase the codes from the PROM chip to reset the computer. After all the repairs you've done to the EGR system it'll be the wildest stroke of bad luck if it showed up again which means theres a computer problem. Disconnect the battery for 5 minutes to erase all memory codes, ( if you got radio codes its a good idea to record them before disconnecting the battery) re-connect the battery and then take your truck for a test drive on the highway. The EGR opens at highway speeds so if there is a problem you would feel it right away that there is lack of power on top end acceleration. 

Unless the EGR circuit to the computer is fried it will be the only reason for code 32 to come on, but we will only know this as long as the codes are extracted. 


A transmission that is slipping will never set an engine code especially for a 94, but what it can do is fool you into believing there is something wrong with the engine especially on acceleration. 

Q: Did you follow the diagnostic flow chart for the EGR circuit , if so what were your readings ?


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## redchevyz71 (Dec 17, 2010)

What exactly is the egr circuit flow chart? Is that the voltage readings that the solenoid should have? But I'm goin to unplug the battery tomorrow and try that


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## octaneman (May 13, 2009)

I've posted 2 files for you to look at on post No.#27, one is a wiring schematic on the EGR showing you how it works with the pin outs to the solenoid plug and the computer. The other is a float chart that shows you how to diagnose EGR problems step by step.


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