Engine Idle

I have a 96 with a stock 5.7 vortec. I'm having trouble with it idling. When I first crank it, it idles at 2500 rpm for probably 20 seconds and then idles down to about 1100. When I put it in gear it will drop down to 700 - 800 like normal.

My problem is it will randomly jump to 2500 rpm and stay there sometimes the whole times I'm driving, then randomly idle back down whenever. There is no pattern to this. It's getting to be a PITA to try to drive with this happening. It's hard to be stopped and kept it still when it suddenly jumps up to 2500 rpm. It will drive at 35 mph while idling! It stays at 2500 rpm more than it idles normally. It seems to be happening more regularly too.

I replaced the two electronic parts on the throttle body (I don't remember what they are called, one is maybe the throttle positioning sensor(?). I also took the throttle body off and cleaned it really good. The throttle cable is not sticking. It doesn't have a solenoid with a plunger that controls the idle.

It runs great - very smooth, no missing, backfiring, etc. It also shifts great. It is showing no codes.

I'll post more information as I work on it. Thanks!

far as im aware your supposed to have an iac valve. there was also an idle air control valve on the early 90s and the later 90s tbi and mpfi models. however it sounds like you already replaced it as i think there is only a tps and iac on your throttle body. shot in the dark im thinking egr or sporadic vacuum leak of some kind somewhere.

The tps and iac are the things I changed. I'm going to start tracing out vacuum lines and see if I need to replace any. That is what I was thinking it might be. How do I check the egr to make sure it is working?

Thanks for the info!

vacuum leaks are fairly simple to find if you catch it during a normal operation time. spray around the engine bay with carb cleaner. around the intake manifold and lines and such and if the idle suddenly changes when you spray a particular spot you found the leak. as far as the egr im not sure. i just replace them when i have a problem with them. usually have a cel relating to the egr aswell tho but its not unheard of to have a problem the ecm doesn't think is actually a problem.

I don't have the service manual handy so look up IAC reset.You also never mentioned if you are setting codes or even have short term codes.
I recommend first to clear codes after it is idleing at correct rpm. Shut off truck, disconnect pos batt cable for a minute, then touch it to ground to zero all stray voltage. Reconnect, restart. If you need further help I'll look up factory procedure. Just can't remember it. Good luck. And keep us updated.

Definitely sounds like a vacuum leak. I have seen EGR valves cause problems with a rough idle or hesitation when accel but a high idle like you described would be new to me unless that is the source of the vacuum leak. I not only like to search for vacuum leaks with the water test, like @drakon543 said, but I also use my mechanic's stethoscope to listen for them as well.

There is no reset for the IAC valve but the port it slides in to can get badly carboned up and I always clean that with carb cleaned and a small round brush. Hard to see in there except with a mirror and a light.

PnwVietvet- I stand corrected, missed the cutoff year. I was thinking (a problem I know) that 2 barrel TBI until 96.
Wouldn't a MAP sensor have a hand in this too? If no Vac leaks? That was my original thought that the IAC was out of range so it was having to wait for next time cycle to adjust idle down farther. Without looking at signals I'm guessing at it.
A bluetooth live data should show enough to determine cold start/ egr/ IAC/ MAP/ MAF don't you think?
It could be something simple like a bad engine ground Floating around.










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https://www.rockauto.com/en/moreinfo.php?pk=678421&cc=1061356&jsn=1873

https://www.rockauto.com/en/moreinfo.php?pk=1298890&cc=1061356&jsn=1890

It would be easy enough to check by just doing a MAF clean and see if it effects the outcome. Could also be a vacuum leak at the MAP plug in. Hell at this point anything is possible.

What is confusing to me is that he says NO CODES. I would assume codes but hell, who knows.

My very first thought was a vacuum leak effected by engine heat or the IAC.

PNW VietVet said:

https://www.rockauto.com/en/moreinfo.php?pk=678421&cc=1061356&jsn=1873

https://www.rockauto.com/en/moreinfo.php?pk=1298890&cc=1061356&jsn=1890

It would be easy enough to check by just doing a MAF clean and see if it effects the outcome. Could also be a vacuum leak at the MAP plug in. Hell at this point anything is possible.

What is confusing to me is that he says NO CODES. I would assume codes but hell, who knows.

My very first thought was a vacuum leak effected by engine heat or the IAC.

Click to expand...

when i did my last big fix before selling my 97 i forgot a large vacuum line and it allowed the rpms to climb pretty far but i didn't get a code. i didn't let it keep running as the rpms were higher than his issue.

Believe it or not, driving it yesterday and today - no problem,. Idled normal the whole time. As I said, it is not throwing any codes. When I removed and cleaned the throttle body, I cleaned the IAC port too.

I'm going to spend what time I can this week checking for vacuum leaks and replacing vacuum lines. Many of them are old and need replacing anyway. If I don't find anything then I'm not sure what my next move will be.

Anybody else have a love/hate relationship with their tahoe? At least one day a month I threaten to sell it for whatever I can get for it. The other 29 days I love it!