94 Suburban 5.7L periodic searching for idle while stopped at stop lights

[fancyTBI]

My truck does this as well. It is frustrating and I’ve yet to check fuel pressure. I replaced the IAC and did the re-learn and it seemed to help a bit. Mine will do it when it’s hot and I start it and sit and idle. It will jump from 650-1000 until I drive about 1000 feet(estimate, driving just clears it up quickly) or blip the throttle and it will find idle. After driving it and coming to a stop it idles fine. I’ll be referencing this thread to help fix my issue.

 

[Erik the Awful]

It's been my experience when they hunt for idle like that, yeah, the base idle needs to be adjusted.

I disagree. If you're adjusting base idle, you're crutching the system to hide the problem. Fix what's wrong. The OP has already stated that he's re adjusted base idle. The problem is elsewhere. A hunting idle would steer me to checking thoroughly for vacuum leaks.

 

[HotWheelsBurban]

TPS often goes bad around the same time as the IACV. At least this is my experience with the Vortec. If it hasn't been changed, might be worth a look.

 

[DerekTheGreat]

I disagree. If you're adjusting base idle, you're crutching the system to hide the problem. Fix what's wrong. The OP has already stated that he's re adjusted base idle. The problem is elsewhere. A hunting idle would steer me to checking thoroughly for vacuum leaks.

I do not, given my experience with my own trucks. If you follow the procedure listed in the manual to close the IAC, verify you do not have vacuum leaks or pirate air then I'd verify base idle. If it's low, I'd adjust it. 650rpm seems pretty high for base (What OP adjusted it to). So if a person does that and still has a problem, you leave that in spec and then move to the next possible cause. If the thing is hunting for idle without the IAC adjusting counts or disconnected and we've already verified it's not pulling in pirate air, I'd look elsewhere. Like Schurkey said- it's time to look at the datastream. I'd like to verify fuel pressure when it starts acting wonky, look at MAP values, see if it's popping in & out of closed loop and look to see what the O2 sensor is doing when it runs like that. Would be nice to also verify the TPS isn't reporting bogus info to the ECM as well. Schurkey has also posted somewhere else about how to check for a misfire and how to gauge each cylinders' contribution by slipping small, blunted nails under the plug boots at the distributor cap and grounding each out individually. I'd try that chit and see where it got me. Another time I had an erratic idle, it was caused by the TPS. The idle would hunt, or just stay a bit higher and then drop. Step on the gas, ease off the clutch in gear and it was fine. I connected my brick to it, took it on a road trip and got it to act up. Sure enough, the TPS value was changing despite my foot being off the throttle. The IAC opens a bit when it does this. Replaced the TPS, problem solved.

 

[HotWheelsBurban]

I do not, given my experience with my own trucks. If you follow the procedure listed in the manual to close the IAC, verify you do not have vacuum leaks or pirate air then I'd verify base idle. If it's low, I'd adjust it. 650rpm seems pretty high for base (What OP adjusted it to). So if a person does that and still has a problem, you leave that in spec and then move to the next possible cause. If the thing is hunting for idle without the IAC adjusting counts or disconnected and we've already verified it's not pulling in pirate air, I'd look elsewhere. Like Schurkey said- it's time to look at the datastream. I'd like to verify fuel pressure when it starts acting wonky, look at MAP values, see if it's popping in & out of closed loop and look to see what the O2 sensor is doing when it runs like that. Would be nice to also verify the TPS isn't reporting bogus info to the ECM as well. Schurkey has also posted somewhere else about how to check for a misfire and how to gauge each cylinders' contribution by slipping small, blunted nails under the plug boots at the distributor cap and grounding each out individually. I'd try that chit and see where it got me. Another time I had an erratic idle, it was caused by the TPS. The idle would hunt, or just stay a bit higher and then drop. Step on the gas, ease off the clutch in gear and it was fine. I connected my brick to it, took it on a road trip and got it to act up. Sure enough, the TPS value was changing despite my foot being off the throttle. The IAC opens a bit when it does this. Replaced the TPS, problem solved.

When TPS was bad on my Burb, it was doing this "hunting around"for idle speed, very inconsistent. Replaced the part, problem solved. Apparently this is a common enough symptom, that I didn't have the full description to my cousin ( who I was trying to get diagnostic information over the phone from; he lives out from Austin) before he suggested that part was at fault.