Hi all,

I just picked up my '92 LT1 coupe this weekend. The car was with my mechanic last week who was, among other things, troubleshooting a high idle situation. He ran smoke through the engine and found no vacuum leaks, so he focused on the Idle Air Control valve. He removed it, cleaned out a bunch of carbon build-up, lubed the "plunger" as he referred to it (I believe he was talking about the pintle valve) and put it all back together. The high idle seemed to be gone, so I brought the car home Saturday afternoon.

I put about 100 miles on it Sunday. The car ran great all day, but near the end of the day the high idle came back again for a while. It went away after driving it a bit more, perhaps 10 minutes or so in stop-and-go traffic. My wife and I stopped for dinner and the car sat about 40 minutes or so in the parking lot. When we came back out, I started the car and now it seemed to be idling very low (about 500 rpm or so in park, and 400 in drive). We drove the three or four blocks from the restaurant home. My wife jumped out before I pulled the car into the garage and as I was pulling it in, she noticed that the garage was filling up with a thin white haze that was coming from the 'Vette's exhaust. I jumped out expecting the smoke to smell like burning oil, but it smelled more like a super-rich mixture condition. Almost "carbon-y", if that's a word. I shut it off, restarted it, tried clearing out the engine a couple of times, but the white smoke continued to seep out of the exhaust. I shut it off and called it a night.

I haven't started the car yet today, but I'm thinking that the smokey exhaust was just a by-product of the extremely low idle condition (too much fuel loading up the engine). The car has 62k miles on it and wasn't exactly babied by its' previous owners.

I'm wondering if I might be better off at this point simply replacing the IAC completely, or is there anything else I might check before I run out and start throwing money at it.

Any thoughts would be sincerely appreciated. My mechanic is experienced and quite familiar with Corvettes in particular (he does a lot of work for a local Corvette-only dealership and drives a '79 himself) but since this is a "hobby" car, I'd like to try doing as much of my own work as I can.


Thanks!

Ron

PS - My FSM was just ordered today, so it's on its' way. But I just thought that I would post here ahead of receiving that.

 

Seems like you had a knowledgable person work on your problem, so I don't suspect it's anything he did or didn't do.

A few years ago I repeatedly read that the IAC can't be "cleaned", but I cleaned mine with excellent results. The only "trick" I used was to be sure I pointed the pintel downward as I sprayed it with brake cleaner to prevent the spray from entering the motor area.

Years later I did change both the IAC and TPS on my L98 when I bought a new TB. As I recall, the new IAC came with a new gasket, too.

Based on what you wrote, if it were me, I'd replace the IAC with a new one and would specify one that GM used on the original engine - ACDELCO? - I'd also ask/verify that the IAC opening in the TB was cleaned too.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!

 

Thanks Jake!

 

rough idle, driven hard, white smoke = head gasket........

 

I have been cleaning IACs for 19 years on several different cars. My long term success rate was real low as it might be good for a short time.

Now I don’t even mess around cleaning them anymore except for diagnostic purposes. I just replace them and be done with it. Ever time I installed a new one, I was fixed.

 

I'm thinking it's time to swing by the parts store and pick up a new IAC. I can at least eliminate that potential issue.

I drove the car again last night. When I started it, it started idling at about 500 rpm right off the bat. I actually went through the IAC reset procedure (start car with slightly opened throttle, run for 5 seconds, shut off for 10 seconds, start again). After doing that, the car started and idled at just above 1000 rpm (still cold at this point) and after it warmed up, it now idles at around 500 in park and around +/- 400 in gear. When in park it seems fairly smooth, but in gear, it now has sort of a rythymic surging while idling. I had a small amount of the rich-smelling white smoke at first (much less than the previous night), but after driving it for about 10 minutes or so (and clearing it out a few times) that went away.

Where should a well-tuned LT1 idle when cold, when warm and in park, and when warm and in gear?

Thanks too for all the replies!

Ron