I bought my wife a 1991 L98 Corvette coupe back in February 2010. It ran great, looked good. Later on in the year, it started running rough and dying in traffic. I changed the spark plugs and wiring. It helped but still did it. I changed out the fuel injectors and replace the TPS, MAP sensor and went through the vacuum lines looking for leaks. Still does it. I went through the codes looking for something and nothing came up. I'm now thinking about the chip in the ECM and the ignition module as that's only thing that I haven't touched. It gets even worse when the AC is on. Sometimes, it runs great for about 15 minutes and then goes rough. Any thoughts on the rough running?

 

If it only dies at idle, I'd suspect the idle air valve.

 

I replaced that, too. You're talking about the IAC.

 

Wow you must be made of money. Look at all the things you spent money on that were good in the first place.
If you want to spend more money, you should look in the distributor. the problem is either the original dist. coil or the dist. module.
BUT,
sometimes the MAP sensor fails intermittantly. It tells the computer in conjunction with the TPS if you have your foot on the gas or not, and if you are accelerating or decelerating.

 

What does the fuel pressure read at idle, WOT and after shutdown?

Have you checked the coil and other ignition parts by the Manual procedure, or did you just throw money at it and replace without checking?

 

I fly jets for a living. Let me ask you a question. If you have a component that is near its service life, do you

a. Keep the component running until it fails or

b. Change it out.

Depending on where you are, it could be a big problem. At 37,000 feet, you don't want old parts in your aircraft that are that close to service life. I take the same view with my cars. Parts are cheap. Life isn't. This process has been going for over a year now. My wife drives this to OKC and back. I want to make sure she arrives and returns.

 

Replaced the injectors with what?

You replaced the IAC but what else did you do? I have never just replaced the IAC. I check timing to be correct, make sure the IAC passages in the TB are clean and take off the IAC housing to make sure it is clean. After that, I reset the IAC and check the TPS.

Assuming your fuel pressure is fine and the injectors are good, I would hook up a scantool to see what your IAC counts are with everything off but the engine running.

 

Have you checked the FP when it's hot? It could be the fuel pump acting up once it gets hot. That's how mine acted except it wouldn't run once it got hot.

 

And I test jet engines and rockets that kill crazy muslims for a living, but I dont throw money at problems when the money could be better spent on replacing a bad wheel bearing before its service life ends.

You need to be more specific about exactly what has been checked out.

 

Basically it is either a fueling or ignition issue. First steps would be to measure fuel pressure and then check ignition timing. If these pass, check your plug gap. Another easy check would be the catylitic converter. Use temp gun to see if any of the three cats are clogged.

 

Having spent over 30 years as a licensed aircraft mechanic, a Technical Specialist and a Manager of Technical Operations for a major airline (which by the way is headquartered really close to Grapevine) I can appreciate and applaud your position regarding a sound maintenance program, and the concern for life, be it for aircraft or vehicles, but in reality, some parts are time tracked and some are "fly to fail", thus the reason, for the MEL. (For the non airline types, that is the FAA approved list of things that an airliner that are allowed to be broken and still have the aircraft safe and legal to fly.. Kinda like driving around with a headlight burnt out or the CEL on)..

That said, from what you describe and what you have replaced, I would look at either the ICM or the coil. Both can break down due to heat/age, etc...

 

Dear Okie,

I had a problem with my 1991 about 6 months after I bought it used. I replaced a few parts but it kept getting worse. It wouldn't run when it got hot. I took it to a Chevy dealer here in New Jersey that specializes in Corvettes. I guess my 1991 was too old for them. ( This was 3 years ago.) After 3 days and $275.00 they thought it might be a vacuum leak and would cost $1,500 to replace all the intake gaskets. I took the car home and replaced all the gaskets. Still no change. I kept replacing parts and the computer had no codes. I checked the ohms on the injectors and had 4 bad readings. After I replaced the injectors the car ran good.
The moral of the story is - I replace parts until I find the problem. It's cheaper than taking it to the shop and the parts that I replace on my 20 year old car won't fail for many years to come. I have only spent about $300 over the past 4 years and I have many new parts. One trip to the shop and I could have spent thousands and only have a few new parts replaced. My friend calls me a "Parts Jockey" but it's cheaper and faster than the shop and most of my fuel and ignition parts are new. I had some water in my tank and it drove me crazy because I was looking for a failed part. Drain some fuel from your tank and look for water. I know your injectors are new but check the ohms when they are hot. Thats when they fail. Also, strong fuel cleaners will disolve the coating on the coils inside the injectors and short them out. Thats how mine died.

Good luck with your problem and keep changing those parts.

Mickey

 

Sorry but the mechanics weren't any good. I specialize in Corvettes or anything you care to name as long as your credit card is good. The only difference between them and you is that you don't bill yourself labor which is why you are "cheaper". Yes, your friends are right. That is about the most foolish way to do a repair. You lucked out on the solution otherwise you'd have done a full rebuild from stem to stern. Your initial problem is that neither you nor the shop were diagnosticians. Any fool can replace parts till the solution hits them in the face. Neither of you had access to a scanner or knows how to use one which is why the solution presents itself by luck.

And how will that tell you if it is spraying well or clogged? Send it out and it can be tested thoroughly.

I wouldn't unless the OP has a deep wallet.

 

this could really be anything so get your credit card out.

Check the head gaskets. Fuel pressure when hot. check for the chip to be locked into the ecm. Check the coil. check the rotor in the dist cap.

Or just buy all those parts and put them in and report back.

How does the car run when you drive it? does it perform like a vette still? When running rough can you WOT it and it goes just fine?

Did you do the AIC reset when you messed with that?
Whats the timing set at?
Cats have any issues?
o2 sensor? exhaust leaks?

What does the wife report was happening the first time it started? did it ever over heat on her? Does she even know anything about that?

Thats about all the generic stuff I can think of.

 

With you saying it runs fine for about 15 minites and then it starts running badly my first guess would be "Ignition control module", or possibly the coil. These parts along with the injectors are very heat sensitive and once hot can start malfuntioning..Running the car with a scanner hooked up is a good way to pinpoint a problem.....Just my 2 cents..WW

 

Fuel filter/pressure.......ign module, fuel inj signal interference from wire rot, poor grounds, faulty ECM or Prom.

Simple. One thing at a time until a result is achieved.

Join a Corvette club and see where members take their cars. It may NOT be the local vette shop...Vettes are NOT the typical Chevy...they're Corvettes and 90% of the car is made of parts that are labled "Corvette Only" and you'll understand that when you feel the price of parts.

 

How do you actually "learn" anything about how the engine functions if you just get lucky changing parts?
Diagnosing symptoms and reacting to the available info is how you learn, and repair a complicated and expensive system.

 

Eventually you will hit on the answer, I suppose. As they say, even a blind squirrel will find a nut sooner or later. Assuming it doesn't starve to death first.

 

I guess throwing parts/money at problems is the same logic that keeps lottery sales so high in recessions when nobody has money to spend.

 

Well, that is where you fools go, the lottery. I have just gotten 5 emails from an attorneys in Nigeria. Well, I'm going to be rich. I will be so rich, when my C4 has a problem, I can pay someone else to throw parts at it. In fact, once all my money comes in, I will be hiring someone to type my posts for me.

Lottery? Why take chances. This is a sure thing. I'll buy Vivid Entertainment and all the girls who work there will have it in their contract to spend a night a month with me. I probably won't even have time for you common folk. I'll be with the rich and famous.

I might even hire someone to fix my cars by Ouija Board. We'll ask the spirits what part to replace. "Oh Great Spirit. My car doesn't start. What to replace?". Answer: Blinker Fluid.