1994 corvette LT1

[mky1969]

Im having an intermittent problem with my 94 vette. It has 55k miles. When I drive it for an hour or so then let it sit for about that long it stumbles and dies when I go to restart it. If I let it sit for a couple hours it will start and run fun. It does fine on short trip and if I stop for just a minute on long trips and restart it runs fine. So far I've replaced the coil, ignition module, MAF sensor, fuel pump, fuel pressure regulator, cap and rotor on the optispark. I'm stumped. Anyone have any other suggestions what might be causing this?

[carsandcycles]

Might be leaky injector(s) giving a temporary flooded condition. Maybe try some injector cleaner (Techron and SeaFoam come to mind) if you don't use said products already; couple of full throttle runs may also help (exercise) a dirty injector. I yield to a higher authority though; good luck.

[mky1969]

Thank you. I put a fuel guage on it and it was holding presure for about 40 minutes. Is it possible the injector/s could leak intermittently? The only other thing I could think of is the temp switch on the water pump giving a false reading but when its hooked to a scanner it reads fine. I've been chasing this for over a year.

[olddog]

It does sound like a heat soak issue with electronics breaking down.

What do you mean by stumble and die?

Does it miss, as in some cylinders are hitting and some are not? If so is the miss steady or random.

Does it putt putt like it is flooded (rich)?

Any popping or back firing?

Does it matter if you hold it to the floor? Does it respond to the accelerator peddle.

[mky1969]

Originally Posted by olddog

It does sound like a heat soak issue with electronics breaking down.

What do you mean by stumble and die?

Does it miss, as in some cylinders are hitting and some are not? If so is the miss steady or random.

Does it putt putt like it is flooded (rich)?

Any popping or back firing?

Does it matter if you hold it to the floor? Does it respond to the accelerator peddle.

It stumbles like its missing out, either flooding or not enough fuel, a couple times if I kept my foot in it, the car would take off and come out of it, Most of the time the condition gets worse after a block or two and dies. I'd say the mis is all over, the motor just doesnt want to run. Saying its a miss might not be the term to use, it doesnt run like its just missing on one cylinder. I havent noticed any backfiring when this happens. The exhaust smells like its running rich.

[mky1969]

any chance it could be the ECM?

[thcardoc]

There isn't just one thing that can cause the reported symptom. What you need is a plan that will make the symptom occur, and a testing strategy that will allow you to narrow your focus as rapidly as possible.

You need to have spark and fuel to get this to run, but more precisely you must have enough spark at the right time, and the correct amount of fuel.
Now the majority of the time you are getting what you need because the car runs fine. But under that hot soak condition something is changing and that is likely the only time that the fault is measurable (detectable through testing).

Your first concern is whether this problem is spark, fuel, or both. When testing for spark you need to see of you have enough (25,000v or more) and to do that you need a spark tester like an ST-125 which looks just like a spark plug with no center electrode and an aligator clip welded to it for connecting to ground. If the engine starts and runs, but spark isn't jumping the test tool, then you still diagnose as "no-spark". Just trying to check for spark with a spark plug or a screwdriver and allowing the spark to try and jump to a ground (valve cover or whatever) can trick you into thinking you have enough spark when in fact you might not.

You also have to confirm not only that you have fuel pressure when the problem occurs, but that your injectors are being pulsed long enough to dispense the fuel to the engine. An oscilloscope, and a low amps current probe are both indispensible tools for this kind of testing. You could have enough connectivity to flash a test light (noid light) while the computer for whatever reason may not be able to actually open the injectors. You have to be thinking about the entire circuit here, power to the injectors, as well as the completed ground circuit through the computer.

Now if you have spark, but no fuel injector command, you get to concentrate on what the computer needs as far as inputs, and the injector circuits.

If you have injector command, but no spark, you're going to be looking at the secondary ignition system.

If you don't have spark and you also don't have injector commands, you're actually looking at the primary inputs for the opti-spark and the PCM distributor reference. So you need to predetermine your testing points, then get the car to act up. I can walk you through to the next step from there if necessary, although you may get your answer to this puzzle right away with a good strategy.

[thcardoc]

Originally Posted by mky1969

any chance it could be the ECM?

Sure there is a chance. There is a greater chance that it is not.
The pick-ups inside the opti spark distributor are easily capable of causing the symptom you have reported. Do you want to just replace the computer, and/or the distributor and then hope you repaired this?

The problem with "swap-tronics" is you never really know if you actually have fixed the car or not. At best you only know that you haven't experienced the symptom again, yet. You could litterally be left with walking out the door each day to drive your car wondering if today is the day you find out that it isn't fixed yet.

These kinds of problems can be solved without throwing a shopping cart's worth of parts at them. It does take patience, training, the right equipment, and a solid game plan.

[mky1969]

Ive somewhat ruled out the optispark since I can hear the injectors pulsing, its my understanding that the ecm has to get a signal from the pick up in the optispark before it sends any signal to the injectors. I had a mechanic stop by and he put it on a scanner. He said all the sensors looked within specs, he pecked on the ecm and said the rpm jumped around on his scanner, I was in the car and didnt notice the tach bouncing. I had a second mechanic look at it and he wiggled the red connector at the ecm and it stumbled or bogged a couple of times. Wiggling the connector doesn't cause it to consistently stumble. Meaning it doesnt stumble every time you wiggle the connector

[mky1969]

Does anyone know of a mechanic in Southern Indiana capable of finding whats wrong with this car?

[thcardoc]

Originally Posted by mky1969

Ive somewhat ruled out the optispark since I can hear the injectors pulsing, its my understanding that the ecm has to get a signal from the pick up in the optispark before it sends any signal to the injectors.

They may be opening but, are the injectors opening up long enough?

I had a mechanic stop by and he put it on a scanner. He said all the sensors looked within specs,

Was this data retrived during a failure event?

He pecked on the ecm and said the rpm jumped around on his scanner, I was in the car and didnt notice the tach bouncing.

The old "tap test" was known to work great at times, and not so well at others.

I had a second mechanic look at it and he wiggled the red connector at the ecm and it stumbled or bogged a couple of times. Wiggling the connector doesn't cause it to consistently stumble. Meaning it doesnt stumble every time you wiggle the connector

That suggests that a pin fit, drag test is in order. However without specifically relating it to a portion of the system during a failure event you could be fixing "A Problem", without fixing "The Problem"

[thcardoc]

Originally Posted by mky1969

Does anyone know of a mechanic in Southern Indiana capable of finding whats wrong with this car?

Sure what town are you in. Heck for that matter one of the best in the country is in Indianapolis.

http://www.lindertech.com/

[mky1969]

The data retreived was not during failure

[mky1969]

Originally Posted by thcardoc

Sure what town are you in. Heck for that matter one of the best in the country is in Indianapolis.

http://www.lindertech.com/

I'm in Seymour, just an hour south of indy right on 65
Just called them, they only work on injectors off car only now

[thcardoc]

Originally Posted by mky1969

I'm in Seymour, just an hour south of indy right on 65

Give Jim a call. He can direct you to someone in your area with experience on these.