# 1991 chevy s10 blazer 4.3 engine backfires in intake manifold



## dkpippin777 (May 29, 2008)

i recently got a 91 chevy s10 blazer from a relative. when you let it idle its ok, but when you try to drive it it starts to backfire and sputter in the intake. it has a TBI automatic 4x4 vin #z . when you give it alot of pedal it runs good but if you let off and try to just cruise then it starts to backfire. ive checked the tps,vaccum lines. ive taken the egr off and cleaned it, seems to have problems, no codes came up so far unless i unplug something. i've seen some comments on here about a ground issue? is it possible that this would cause this to happen.i even went as far as checking all the lifters and clearance cause my uncle was told by some wannabe mechanic that they were stickiing ( i didnt think it was possible but i checked all of them anyway). the timing was checked ,new plugs etc. im very good at repairing things but got stumped when it came to this. my book says to check egr and or the ECM. can anyone help me. ill check whatever i need to fix this thing. its a very nice rig. please help me.. if you want to contact me by email use [email protected] thnx dwayne


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## Raylo (Feb 12, 2006)

Sounds like a stuck open EGR valve. It runs good at high loads because that's when the EGR is open most normally. The EGR should close as the load comes off and be shut at idle.


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## dkpippin777 (May 29, 2008)

how can you tell if its stuck open, if i take a flashlight while its running will i see this happen? and if this isnt the problem is there anything else that could cause this dang thing to do this ? thnx for you reply, dwyne


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## Raylo (Feb 12, 2006)

I'm not sure if there is an easy way to check it visually without removing the EGR and looking at the valve butterfly and cycling it by hand to check for freedom of movement. When the EGR failed like this on my S10 I was out of town and had to take it to a shop for repair. Maybe one of the pros here might have some advice as to how to check. I did have to pull the EGR during a cam swap on my Camaro awhile back and it wasn't a hard job. But I didn't have any EGR issues then so I didn't pay much attention to it.


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## dkpippin777 (May 29, 2008)

i removed the egr and cleaned it.it wasnt very dirty at all. there was no cracks or noticable defects with it.


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## Whitetail Crazy (Apr 30, 2008)

If it idles OK then the EGR is probably not sticking open. I hate to have you just changing parts but it sounds like it could be the ignition module or possibly the pick up coil in the base of the distributor.
Also, if this is the diaphram type EGR, reach under and push the diaghram up while idling, that will make it run rough or possibly even die. If no change then you may be looking at an EGR problem but it really doesn't sound like it.


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## Raylo (Feb 12, 2006)

I missed that part about idling OK. Whitetail is correct if it is idling OK probably not the EGR.

Go figure, I finish painting the roof of my truck and take it for a spin and I am getting the same symptoms as when the EGR died 3 years ago. Mine sounds like yours except it'll hardly idle at all. I pulled the Haynes manual and it says my CMFI 4.3 with auto trans has a computer controlled EGR that has a pintle that opens more or less IAW PCM signal. No way to test or visually tell if it is bad. Haynes says it requires "special electronic test gear". Guess I'll have to go find a new one... or perhaps just fashion a plate to block it off.


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## dkpippin777 (May 29, 2008)

it also has an electronic egr witha small filter.that 2 vacuum lines going to it and the electrical plug. i really need some help here guys lol. ive got up for sale on a craigs list if you want to see it. http://redding.craigslist.org/car/704004281.html. im asking $900. obo.. im driving it still.


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## Raylo (Feb 12, 2006)

That's different from mine, which has no vacuum lines. It apparently uses a small electric stepper motor. So you might have a code Z motor?

Anyway, the only obvious things I can think of to cause backfire are egr, swapped spark plug leads, or timing way off, as in a slipped timing chain. But all of these things should be noticeable at idle. Sorry, without more info I can't help.

Good luck.


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