1991 5.7 TBI runs rough when cold

[camspeed01]

Seems to be normal to me. Like Sonny mentioned, every time I look at the timing using a light with the ESC wire plugged in, the timing is usually really advanced at idle for some reason, sometimes erratic. Neither of our trucks miss at idle regardless of temp, its just that sometimes they idle smooth as polished glass and other times they idle rough regardless of ambient or engine temps. When they're idling smoothly, the timing isn't as advanced. Why the ECM commands this is beyond my comprehension.

I don't know why mine is so bad then, it's not even very cold out, roughly 70 degrees, and on cold start the idle is so bad it will stall if I don't give it gas.

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[DerekTheGreat]

Compression test? Leak down test of the coolant system? How does the spray patter look coming out of the injectors? Did you replace them when you rebuilt the TBI? Gaskets OK? Play in the throttle shaft?

 

[camspeed01]

Compression test? Leak down test of the coolant system? How does the spray patter look coming out of the injectors? Did you replace them when you rebuilt the TBI? Gaskets OK? Play in the throttle shaft?

This is actually my first Chevy so I haven't rebuilt anything yet. I haven't done a compression test because it seems to be healthy after it warms up and has plenty of power. Pray pattern out of the injectors looks ok. I haven't touched the tbi otherwise though it anything else for that matter

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[DerekTheGreat]

The original 350 in my truck had plenty of grunt and power, yet still had a burned exhaust valve in cylinder #1.

Running rough when first started could be attributed to coolant settling in the combustion chamber(s) while sitting overnight.

 

[eran tomer]

Compression test? Leak down test of the coolant system? How does the spray patter look coming out of the injectors? Did you replace them when you rebuilt the TBI? Gaskets OK? Play in the throttle shaft?

i have a bit of play in the throttle shaft. what's the implications of it?

 

[DerekTheGreat]

i have a bit of play in the throttle shaft. what's the implications of it?

Pirate air. It could cause an erratic idle or fuel trim issues. Mine is also worn out and I've noticed I've got a dirty injector at cold start up. Fuel trims are still good while cruising though. However, my IAC counts are down to around 12 to maintain target idle. Two years ago I think they were up around 40 or 50.

 

[MrPink]

I know this is an old post, but has anyone else experienced this issue and found a fix? I just got my 92 k1500, and frankly I have never worked on something like this before. I was able to read the codes and got code 43, referring to a knock issue, which sort of relates to the last 2 posts on this thread.

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my truck is also throwing a code 43 but i have not narrowed it down yet. i need to unplug the EST wire and see if that changes it, if it does then i have an issue with the timing.

 

[Schurkey]

I was able to read the codes and got code 43

Fix the code(s), see what happens.

I'd be looking at the data stream, see what the sensors and outputs are doing.

 

[DerekTheGreat]

my truck is also throwing a code 43 but i have not narrowed it down yet. i need to unplug the EST wire and see if that changes it, if it does then i have an issue with the timing.

Hook a scanner up to it and watch for knock counts. Seems to be a common problem, the knock sensor. So if you don't see any knock under throttle, that's an avenue to explore. The ECM will self test it once per key on cycle. If it doesn't see knock under normal driving, it'll try and advance timing until it does. It'll advance it to a certain point and then stop if it doesn't see knock and set code 43. Was the issue with my '89 and wife's '92. In her case, the wire to it was cut and in mine, the sensor was bad.

 

[MrPink]

my truck is a 95, cant just hook a scanner to it. and i unplugged the EST wire and it ran considerably worse. so i am picking up a timing light today to start with that.