Merrill94vette

Need expert help on this one. My 1994 Corvette will run great until I shut it off, wait 5-10 minutes, then restart. Upon hot restart, engine (LT1) runs badly with misfire/backfire problems. Will run great for extended periods of time as long as I don't shut it off, wait a short period of time, then restart. Once it has cooled back off, runs great again. 43K miles on car.

MarkRixey

Could that be due to the ominous optispark problem with c4's before 95?

FAUEE

Don't let people jump to the opti, it doesn' really sound like an opti issue anyways.

Does it satay running poorly after restarting? Or does it right itself after a bit? You could have a fuel leak either at an Injector or regulator. It could also be a vacuum leak, ypur ignition coil, or the ICM.

you need to get a fuel.pressure gauge on the Schrader valve and check fuel pressures.

FASTAZU

You have to start with the basics.

Paul Workman

The symptoms, especially if (as mentioned) it straightens out (more or less) after running for a few seconds (or after blipping the throttle to "clear" it), are typical of fuel leaking into the plenum via leaking injector(s) or fuel pressure regulator.

Liquid fuel in the regulator vacuum hose indicates a ruptured diaphragm. (Normally, fuel pressure at the rail w/ the engine running should be ~40 to 42 psi for the LT1/4 motors.)

A fuel rail pressure leak down test will indicate (among other things) if one or more injectors are leaking. Monitoring pressure at the rail after first turning the ignition switch to ON long enough to pressurize the rail, and then turning it OFF, pressure should hold to within a pound for up to a minute or so. Point is, if pressure immediately bleeds off several psi within a minute, then it is likely an injector (keeping in mind those specific symptoms you described).

Merrill94vette

Don't believe it is the opti either, due to the fact that it will run great all day until you shut it off, wait 10 minutes, and then restart the car. I have checked for vacuum leaks ( found one earlier that was causing the A/C not to blow out the front vents, only the floor and the cruise control not to work. This was the vacuum source for the fresh air door/cruise) No other leaks detected. I have hooked up a fuel pressure gauge when cold, and it was reading good. I am hesitant to hook the pressure gauge up to the Schrader valve when hot due to the possibility of leaking fuel onto an already hot engine. Replaced the fuel filter earlier on and that seemed to have fixed it for a while, but it is now back. This started occurring this last time after topping off the fuel tank.....ran great there, but barely made it back to the house. Almost seems like some kind of vapor lock may be causing this. Once it happens, it seems to stay and gets worse at low idle when going from park to drive. When revving up the motor I am getting occasional backfire and black smoke from the tailpipe. I have ohmed out the injectors at hot & cold and they look good also. I have also replaced 1 of the 2 O2 sensors with no help. Seems like some kind of fuel issue to me. Is it safe to leave/install a fuel pressure gauge to the Schrader valve so that I can check fuel pressure when this happens?

FASTAZU

Yes you can "leave/install " while running just be sure of its location in relation to any moving parts.

This may be a long shot, I have seen the small line in the tank that hooks the pump to the fuel line get lose. the plastic clamps and hose will swell over time. you will have a loss of fuel pressure, more when the fuel level drops below this point.

Smokeysvette

Have you checked the ICM that sits near the coil on the front of the engine? They have known to cause havoc too. Mine on my coupe acted sort of like yours and took a shot at it and fixed the problem. Most of the time when these go out the car acts fine until under a hot soak condition then doesn't like to start and then sputters and just quits and won't start again. Also it never set a code.
Another thing you might check is the Prom this also will cause real havoc too and at least with me didn't set a code either. This happened to me on another '92 vert
Please like other have said, do the basic checks first. Then if all seems to check out OK then you might start checking the things I have suggested.

MarkRixey

This still sounds like a bad capacitor. Just sayin'

Merrill94vette

Thanks for all your inputs...... gives me a few things to try. 1) monitor fuel pressure by leaving pressure gauge hooked up during driving to check if fuel pressure changes after sitting after running. (I have checked it cold and it is spot on at about 42lbs) 2) ICM changeout 3) Prom??? Need more info on where this is located and how to check it. Just a note... you would think that when it starts running badly that it would throw out a code, but no error code is being generated.

Paul Workman

Really? Do tell...

Smokeysvette

I'm glad to have helped with anything I might have added to the other posters good suggestions.
The Prom (also called by some as a chip) is located in the ECM and controls all sorts of things with drivability problems. Even sometimes controls the shifting points which it did mine. It also made it run almost the feeling you get when the converter is stopped up.
Not sure how you can actually check it and hopefully some one else can help you there. I just had a spare one because I bought one cheap on Evilbay because there like finding chicken teeth on the 92's. I took the old one out and replaced it and it drove like a complete different car.

Merrill94vette

Just an update... replaced the ICM and have not had it happen since. I think that was it. Thanks for your help.

Renfield

I did that too after similar symptoms. Two months later, my issues returned (won't start again after shut down, fires up the next day.) And, BTW, misfiring/backfiring IS one of the signs of a failing Optispark.

Turns out the ICM was good, as was the coil. Both OEM units were tested and passed. Now I go back into the two year old Optispark...

You can test all day, but intermittent problems dog the Opti.