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Discussion Starter · #1 ·
Hello Forum:

New member here. Just purchased a 1991 Sierra with 167k on it. Don't know the history of the vehicle; it was an auction buy. Here's my problem: When I purchased it, someone had done a lot of recent work to it and replaced a lot of parts. Also, I found out that the cat was removed, and that it had been unregistered, and assume sitting for 7 years.

Immediately, I found out it had problems. We quickly diagnosed that the ignition control module was bad and sending it into a severe firing order. Sometimes it would backfire, but mostly just shut down without warning. We replaced that, and most of my problems went away. Here's where it gets tricky. It would still idle rough and die at times, so I started running fuel cleaners and the problem improved slightly. The next step, we started replacing parts, and have done all the following things and the problem continues.

1. Replaced the Idle air control
2. Throttle position sensor
3. Map sensor
4. Fuel injectors
5. Fuel pressure regulator
6. Oxygen sensor
7. Electronic spark module

I took it to a neighborhood mechanic and they checked the EGR valve and the fuel pressure. Both tested fine according to them. They did a block test and said the head gasket was leaking. (exhaust getting into radiator) Although there may be a slight leak...I'm not convinced that this is the real problem. Today, I pulled my plugs and did a compression check. All 8 were very consistent with around 130 psi. Plugs had about 3k on them and were already getting heavy carbon deposits. Vehicle always smells like its running rich to me. I've gotten the following codes: 33, 34 and 43. 33 and 34 are consistent with the rough idle and associated with the map sensor. 43 states the electronic spark. (is this a timing issue?)

Can't seem to figure out why this is happening. Perhaps my engine control module (brain) is not functioning correctly. Someone has been in there as well, because the back of glove box is missing and the computer wasn't secured properly. So the previous owners were looking at or playing with this as well.

My next step was going to drop the fuel tank and clean it, even change the sending unit and fuel pump while down. Hoping someone on this forum works as a G.M. mechanic and can give me some badly needed advice!

Thanks!
 

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try misting some starter fluid around the engine when its running and see if you get some revving. It sounds like its on the air side, so either vaccuum leak or air filter problem. If you get revving with the starter fluid in certain locations then its probably a vaccum leak thats sucking the starter fluid in. You can get a vaccuum leak around your manifold so if your heads are leaking you might also have intakes leaking. On these trucks though its not that bad of a job, everything is right there and the kits have the torqe settings and you just pull apart and reassemble with the new gaskets.

When you replaced the TPS did you take the throttle body off and really clean the inside well?

If you dont get any revving then i would check the distributor and the plugs and wires. But if you said alot of work was done that could have been one of the things replaced allready.
 

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Discussion Starter · #3 ·
I wasn't sure on the cap, rotor and wires, so I went ahead and replaced them, along with a new set of plugs. Also, I replaced the coil which was the original. Problem went away for a short time, but has returned! I did not remove the throttle body to clean it. I haven't been getting the 33 or 34 codes lately. However 43 code continues to pop-up and that code is for the knock sensor. Have another one to put in...just not that easy to get to! I'll try the starter fluid trick, and see if there's something leaking around the intake.
 
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