Gnawzzy

This car is doing bizarre things that have me going crazy. Maybe someone can point me to the next step

Symptoms: car exhibits intermitent behaviors on startup. i can start, turn off, and restart the car several times in a row and it will do any number of the following things in random order each time:

1) Super high idle and rich exhaust smell. runs and drives fine, but idles around 1800 in park.

2) No idle: will fire and run fine if I hold the accelerator down just a touch to maintain some revs, once I let the pedal go for even a second the revs fall below 300 and the car dies.

3) Completly normal and perfect operation, perfect idle, perfect driveability, everything.

I'm compelled to repeat it will do any one of these things between turning the car off and starting it again immediately, there appears to be no rhyme or reason to any one of these behaviors.

Engine codes: none!

What I've already replace: IAC, TPS, Fuel filter, fuel injectors.

What I'm thinking: Computer, Optispark, or plugs and wires, maybe EGR?

ive gotten to the point where the next things I have to start replacing arent super cheap or easy like the other stuff, ao I was hoping based on this info or any I could provide if you fine folks could give me an idea of which is the most likely to be causing this schizophrenic ossiclation between high idle, no idle, and running perfectly. every time I turn the key it's a crapshoot what it will do.

thanks!

92ragtop

I would suspect the ECM. They can be repaired. However, don't keep throwing parts at it...keep diagnosing.
My 92 had the high idle like you describe. Try this: When the high idle happens, tap on the ECM with something like the handle of a srewdriver. If the idle immediately drops, that is a good clue that its the ECM.

AJT84

My 92 would surge while driving at times, come to a stop and it was running at 1600 rpm if I slipped it into neutral then it might come back to normal or turn off and restart and it would be fine. I chased a few things including the idle air valve. In the end I sent my ecm to SIA Electronics, they reflowed the board and the issue was solved! Definitely try to troubleshoot but I would also GUESS ecm.

Klondike

Check your water temperature sending unit on the front of the water pump. If the connector pins get loose or a bit of corrosion on them it will drive the ecm crazy with intermittent temperature readings. Makes it run rich then lean trying to stabilize the temp for emissions. I damaged mine once when I changed a belt and bent the pins in the plug where they didn't fit together tight. Ran like crap -- then ran great -- then ran like crap again. Took me a couple of weeks to find that problem!

twichy2011

As far as the water temp sending unit goes, if it is failing you would be able to see that on digital coolant temp gauge. I would suspect opti before the computer. I know the computers on the 1992 and 1993 were very problematic, but most people (not saying everyone lol) gets some sort of weird unexplained codes with an ecm failure. When my opti was failing it wouldn't throw a code, but sometimes it would idle at 1400 and sometimes the idle would be roughly 400 hundred and sometimes it would shut off at idle randomly. If I caught it before it died I could keep it alive. What I've learned is that sometimes a failing opti won't throw a code for a while.

Ratboy

Check the maf sensor. I'm sure there are a couple of quick basic tests posted here or online somewhere. Remove and examine carefully all connections to sensors, ECM, and grounds.

twichy2011

The 1992 and 1993 corvettes dont have a MAF.

TorchTarga94

Dumb question but have you checked your fuel pressure when this is happening? I would leave the fuel gauge hooked up and try to replicate this issue. This sounds like a fuel pump or fuel pump pulsator issue to me.

In between starts turn the key on (engine off) and see if you can hear the fuel pump prime. I can almost promise you this is whats going on.

96GS#007

or the fuel pressure regulator.

Also test the MAP sensor and clean the IAC valve. Lastly, perform the IAC reset procedure.

Then again, why do basic 1st grade troubleshooting when you can simply through parts and money at it?

billschroeder5842

For real? I don't doubt you and that seems real old school!

For my education, why would an electrical component behave differently after getting tapped with a hammer as there are no moving parts?

96GS#007

Because it means there's a loose connection on the board....ie a broken solder joint that intermittently losses connection when it's jarred.

However, you typically see this when the car is moving vs sitting at idle.

92ragtop

Cold solder joint or other bad connection in the ECM. The vibration caused by tapping it can identify that. I tried this trick when my ECM acted up, and it dropped the idle every time. No guarantee that it will work in every case though, but it's free to try.