Erratic idle
 

I have a 1990 GMC Sierra 1500 5.7 2wd. When i have the timing advance unplugged my run as great. But when i plug it back in my truck barely runs. Had trouble keeping idle.. Anyone know what could be causing this?

Re: Erratic idle
 

If anyone can help i would greatly appreciate it.

Re: Erratic idle
 

I have been chasing a similar issue with mine for a while and I can tell you no one can say with any certainty what the problem is. It is probably safe to say some sensor(s) is giving the computer some bad information and causing the computer to change setting. But the issue is which one? It's going to take some diagnosis.

Re: Erratic idle
 

Thanks. Im gonna try to change out my knock sensor and see if that works i will keep you updated. Thanks again.

Re: Erratic idle
 

Unless you have reason to think any sensor is bad, your probably going to waste a lot of money replacing good sensors. If you search the internet for rough idle issues you will lots and the few that cured there, all have different solutions. You can find lots of examples of people throwing parts at a problem and still not fixing it. That is why I say diagnosis is important.

Re: Erratic idle
 

Okay thanks. Im gonna look into it a little further.

Re: Erratic idle
 

I've had a similar problem with my 91 but it's a 4.3, it would idle like crap and almost die at stoplights. I replaced most of the sensors and did a full tune up with no results. Then it finally died one day at a stoplight, then again while driving. I ended up completely disassembling the throttle body and cleaning all parts of it with carb cleaner and reassembled with a new gasket kit, some of the passages inside of it were almost completely clogged with carbon. Once it was all said and done it idled like a whole different truck.

Re: Erratic idle
 

That is one suggestion I have not hear before and it makes a lot of sense too.

Re: Erratic idle
 

Im gonna try that thanks 91short.

Re: Erratic idle
 

Post your resorts as I am having a similar issue. Runs great down the highway but when I come to stop after a lot of driving, it starts idling like ****.

Re: Erratic idle
 

If it has an Idle Air Control valve, change that.

Re: Erratic idle
 

RockAuto.com shows this for your '90 with a 5.7 ACDELCO Part # 217437.

I had some idle issues with my 97 and that solved it!

Hope that helps.