Demo man

Hi all.

I have a '92 396 Lingenfelter manual Corvette that would be pure joy to drive, were it not for a serious engine management problem that has plagued the car ever since I've owned it.

It will start up and run perfect when it is cold, but then after it warms up a little, it jumps to a very high idle speed. After a while it will usually (not always) come back down to a still too high, but livable rpm. This is very annoying to me, because the cam sounds great in this car when it's idling at the proper speed.

The other problem it exhibits, is sometimes when I've been been driving a little while, and I make a stop just long enough for the engine to cool down a little: (say I stop a a restaurant and eat lunch), when I get back into the car, it will crank and idle fine, but when I start to pull out, it will spit, cough, and stumble like it's way too lean. It will continue to do this until you floor it under load for a couple of seconds, and then it will return to its normal self. It seems that this invariably occurs at the very most inopportune times (like when there's a cop behind me).

I know I'm not the only one with this problem, because I've seen it mentioned in other forums (always with '92s and '93s), but I've never seen it answered.

I've worked on this thing myself, and had it to several shops. Probably spent at least $2,000.00 trying to get it fixed, but nothing anyone has done so far has provided anything more than temporary relief.

Anyone here care to offer their opinion?

antfarmer2

Have you tried replacing the o2 sensors? And looked for vacuum leaks?

Demo man

Oh, of course... yeah, that and a hundred other common sense things.

Thanks for the reply, though.

Kevova

You may have two things going on. The hot soak restart sounds like it's loading up and your revving to clean it out. Leaking injector or fuel pressure regulator so check fuel pressure see if holds and fuel not leaking in fpr vacuum hose. Warm fast idle vacuum leak most common cause spraying carb or brake cleaner, wd40 etc along intake and throttle body when engine is warm look for idle change.

Demo man

No, it's not rich... it's lean, and the computer is ordering it that way aparently. And we're not talking about "hot soak" here. This is after an hour or so, and the engine has cooled significantly. There are no vacuum leaks, this has been checked by myself and others numerous times, and the injectors have Been replaced twice already since I've owned the car. Who knows how much money the previous owner invested trying to solve the problem.

C4vettrn

Have you done any data logging ?

antfarmer2

and look for a short in the o2 wires. and the knock sensors. Maybe a loose shield setting them off or a plugged or rattling cat.

C4vettrn

If you need a retune talk to Scott. He is a great guy to work with and will help answer questions etc. see his sit below

http://scotthansen.net/

Kevova

Have you driven car with scanner when conditions occur?

Demo man

I know it's not the cats, because I just put two new high flows on there last year, and it did not affect the problem either way. As far as shorts in the wires... well I don't know, maybe you could be right, but you would think that would set a code.., and might be a little difficult (for me) to find.

No, I haven't tried any data logging, as (to answer you other question) I don't have a scanner. I'm not terribly sharp on that particular aspect of mechanicing, but I think someone told me it needed to be OBD3? before I could expect to do that sort of thing. ...then again, maybe that was regarding my 86 pace car, which has a similar issue.

Kevova

The car has obd1. I don't want to be critical. But you don't know about how this stuff works, yet you tell everyone their wrong. If you ask for help you need to be receptive. This is not a production car being able to interpret engine data is critical in determining what to do.

Demo man

Well I admit this has me (and several other professionals) stumped. On the other hand, I'm not exactly some ignorant greenhorn either. I was basically raised in a garage. My dad was the top mechanic in this town, and was always shop forman in whatever dealership he was working in at the time. I spent two years working at a tuneup shop complete with floor dynos myself, but that was in the mid seventies. I'm over 60 years old, and have moved on to more profitable lines of work. Anyway... don't get your feelings hurt. It's just that I (and others) have already tried all the (common sense) procedures that I've seen mentioned here so far, other than the data logging.

I'm betting that there is some experienced soul reading the posts on this board, who has already dealt with this exact issue, who can pinpoint the problem with certainty.

Obviously, that's the guy I'm looking for... and I'm even willing to pay for the advice. I'm tired of wasting my time on this car, and just want to see it fixed.

antfarmer2

Try giving your location might just be a member near you. Have you at least looked for codes with a paper clip?

Tom400CFI

Codes and local members are both great ideas.

Demo man

I am in Chattanooga Tennessee. No, I haven't looked for codes, because the check engine light is not on, so I figured it's likely there are none. However, the shops that I've taken it to, have looked at that in detail.

antfarmer2

Look up Bob2.0 he works at the museum and has some connections. Could not hurt to ask and not that far from you and sure not going to find anyone with better resources.

C4vettrn

What Throttle Body are you running?
Has it done this from day one of you owning it ?
Have you done "anything" to the motor before this started ?

Demo man

Well the TB looks stock to me, other than the little airfoil thing that's been added to streamline the entrance, and the Ligenfelter nameplate on top. I've had it off, taken it apart, and cleaned it thoroughly (someone else's suggestion) and didn't see anything I wouldn't have expected on a stock Corvette.

And yes, this problem was there when I bought it. After one shop changed the injectors to higher flowing units, the problem seems to go away for a couple of weeks, but then it returned just like before.

antfarmer2

Might just need to be programmed right.

antfarmer2

Try disconnecting the battery to reset it then see how it runs then drive it 50 miles and see if it changes.