bwgas

Help! My 1992 Vette idle surges periodically after Vette is completely is warmed up. This happens after a 10 or 15 mile drive. It seems to do it after the engine reaches 200 to 230 when parked in idling. It starts up with no problem, idles perfect when cold before warms up, does not miss or surge while running (hot or cold). I have replaced PCV valve and oxygen sensors. This has not helped. When I take it out on a run with normal driving and pull into the garage, the temp is about 198, let it sit and idle without the air on. The temp climbs to 228 like normal, fan kicks on and cools it back down. After about 5 minutes the engine RPM will go from est. 750 rpm to 1000 rpm intermittenly. The longer that it sits and idles the more frequent it happens. Any ideas?

85 500 horse

If I had to take a guess, I'd say that one, possibly more injectors are fouled.

89sleeper

If that were true why wouldn't the car surge all the time and not just at a certain temp?
It sounds more like a vaccum leak to me!

RRT vette

I agree with sleeper... vacuum leak somewhere.

bwgas

Thanks i will try to find the vac leak took it out today ran great but at stop lights you better keep your foot on the brake or it will take off on you It wants to gooooooooooo

RRT vette

Vacuum leaks make the idle go up and down usually at idle. You can find it by using propane very carfully around the vacuum areas. Don't concnetrate the propane in one area too much. When the propane enters the vacuum it will make the engine idle up a bit. Almost not noticable if the leak is small. You can listen with a stick or hose to your ear around the engine bay and will be listening for a hissing sound, more than likely, when the engine idles down. There might another problem as well. The last statement you made sounds like the minimal idle is a little high or the IAC needs cleaning/replacing along with cleaning the TB.

Is your SES light on?

bwgas

Thanks no lights on, all ok the car had little use last 2 years prev owner less than 500 miles use last 2 years it has 47000 miles on it may be needs a good cleaning??? throttle body????

RRT vette

It never hurts to clean the throttle body especially if you've never did it before. This is what I consider a maintenance item just like the air filter and oil...just not as frequent. If it is dirty, you'll be suprised how much better it will run afterwards. I suggest removing the IAC and TB for a good cleaning of the air passages. I used throttle body cleaner and a old toothbrush. Make sure the gaskets for both look good before installing or this will cause a vacuum leak.

bwgas

I was thinking that it could possibly be the opti-spark. Having carbon tracks causing the rough idle, is this a possibility? If that was the case, wouldn't it idle rough when it was cold before it is warmed up? It only idles rough when it is warm. With an opti type of problem could it be when it is warm but not when it is cold? I am hoping that it is not the opti.

onedef92

My money's on TB/IAC cleaning and/or vacuum leaks, too. Check the C4 TechTips Section for info.

bobmic93

I had the exact sitsuation happening on my car e few yrs back. Hunting idle only when it goes into closed loop. I replaced everysensor possible and it ended up being the opti.

94ZR1

Iac

bwgas

I think you may be right on the opti I check for vac leaks can not hear any leaks and also look over tb and it looks clean Today i ran it in the garage for 35 min and when it fully got up to temp and the cooling fan was coming on at 228 and cooling off the engine then it would start to run up and down on rpm but not all the time but contiune as it idle for the 35 min i also during this time it just stop i had to re start the engine frist time for that maybe opti slow death ???

bogus

I am on the opti, too.

It's a classic symptom.

I would not be surprised if there was a high rpm miss, too.