High Idle Nightmare - '95 Chevy G20 - Code 35

[blisswizard]

Hey everyone,

First time posting on any forum after spending the last month reading most of them researching this issue to no avail. I have a 1995 Chevy G20 Gladiator - 5.7L 350 TBI that i LIVE and WORK out of so this issue is really messing up my whole life at this point. Any help would be much appreciated.

Last month it began to have an extremely high idle. I started to notice needing to apply way more pressure to brake to get it stay put at stop lights and stop in general, the shifting between drive and park and reverse almost became a somewhat of a slamming into gear, and throwing it in park or neutral from drive shot up the idle like crazy. When the issue really became apparent to me it actually would still start up and idle normally, but once i gave it gas and started moving it's like the idle would never return to normal levels. It will get up to 25 - 35 miles an hour on it's own without me even touching the accelerator.

It throws a code 35 {idle air control circuit} and previously was throwing a code 32 {egr fault} as well. I replaced the IAC valve and the TPS first. The code 32 went away but now it still throws a code 35.

I have replaced the following :

IAC valve, TPS, ECT sensor, 02 Sensor, EGR Valve, PCV valve, MAP sensor, Several Vacuum lines, Spark Plugs, Throttle Body Gasket first, then actually rebuilt the throttle body and replaced all o rings and gaskets...

All to no avail. Still throwing code 35. I have sprayed for vacuum leaks and found no change in the idle. I have taken the IAC valve out and plugged the port w my finger and it does not seem to change the idle very much. At this point i am wondering if my intake manifold gaskets are blows as i don't think i have actually sprayed around the whole base of the intake manifold but from what i've read if those gaskets are bad and vacuum is leaking from there i would experience more than just a high idle.

Something to note about the spark plugs, when i put the new ones in i think i forgot to connect one or two of the distributor lines to one or two of the plugs, turned it over and it wouldn't start. Realized i had made that mistake, reconnected them and it still wouldn't start. Ended up putting the old plugs back in and read somewhere that i may have flooded the engine trying to turn it over so many times to no avail, so the first time i went to start it after putting the old plugs back in, i started it with the gas pedal pressed all the way to the floor. This caused a large stream of fire to shoot out of the throttle body. I suppose that is called a backfire? Anyway, now it starts and runs again but there is a hissing sound coming out of the throttle body that is very noticeable with the air filter off the throttle body... not as noticeable with it on but still there.

Now it idles high even on startup. I have only been driving it on short drives daily maybe 6 to 10 min of running or so. Recently this year i have replaced the brake booster, power steering pump, power steering gear box, and had the transmission rebuilt. After transmission was rebuilt i did notice it was shifting between P, D,R more aggressively than before but the guy told me he put a boost in the transmission to up the pressures... whatever that means. This is the 2nd time he rebuilt the transmission in 3 years.

At this point im ready to replace the intake manifold gaskets but i'll be devastated if it doesn't fix the issue as it's a pretty intensive job for someone like me who isn't actually a mechanic, just a very capable and determined DIY guy.

Anyway, any help would be appreciated. I can't make money without my van working and i'm going broke trying to fix this thing.

Thanks much in advance

[MountainManJoe]

welcome to the forum

It throws a code 35 and previously was throwing a code 32 as well. I replaced the IAC valve and the TPS first. The code 32 went away but now it still throws a code 35.

Did you look up what the codes mean? If so , please post it here.

I have sprayed for vacuum leaks and found no change in the idle.

this works fine in a pinch, but to test for vacuum leaks thoroughly, you should loan a smoke tester from the auto parts store.

Did you clean out the throttle body and throttle plate?

[blisswizard]

Code 35 throws when there is a problem w the idle air control circuit. From what i've researched, replacing the IAC valve or some of the other components i listed that i have replaced usually fixes the issue.

I blew plenty of cigar smoke into the air intake for 5 or 10 minutes as well and didn't see any coming out anywhere. I suppose i could rent a smoke machine but never came across that as a method when doing my research.

To my knowledge i cleaned the throttle body thoroughly as i stated i rebuilt the whole thing and replaced all gaskets and o rings etc. There was no real carbon buildup anywhere inside either from what i could tell in the first place.

[blisswizard]

I should add that before this high idle started, after i got the van back after the transmission rebuild, i noticed at stop lights that the van would buck or jerk randomly, almost to the point one time where i thought someone had hit me. Would not happen consistently but maybe once or twice during a 20 or 30 min drive through the city. I also was noticing the idle go up and down slightly, sort of an uneven idle that i had never noticed before. I have no felt that buckling, or jerking since replacing some of these parts but the idle has gotten significantly higher than when that was happening also.

[MountainManJoe]

I have taken the IAC valve out and plugged the port w my finger and it does not seem to change the idle very much.

well that would indicate the IAC system is not really doing its job. Since you replaced the valve already it's probably not the issue, and the problem could be in the wiring. So inspect for chafed wires, corroded connector pins, etc. Measure resistance back to the VCM. Pray that it's not a faulty VCM

I think the IAC pintle is supposed to move out when you turn the key on, and back in when you turn key off.

more info: https://nastyz28.com/threads/any-way-to ... -gm.82908/


1996-97 Vortec 5.7L electrical info: https://lt1swap.com/1996_vortec_5.7/index.htm

[blisswizard]

From what i understand if i plug the IAC port in the throttle body, if it was the IAC valve that was the issue it should barely be able to run or idle at all. As far as the connectors i don't see any corrosion at the IAC port plug pins. Hard for me to make sense of this diagram to be honest. I have heard that in some rare cases it could be an ECM issue (VCM i guess is the same thing). Haven't tracked that thing down just yet i'm actually in another state currently had to fly east for the holidays and leave my broken van to chill for a while. Should be back within the week. Regardless, wherever the wires for the IAC plug run they are wrapped in coating and plastic tube. Don't feel like it's a wire issue and i did pot a voltmeter on the pins to measure some voltage at one point and was getting some reading though not sure what exact reading i was looking for.

[Bret Schmerker]

Could be an air leak bypassing the throttle body and idle air control. Check for cracked air pipes.