John A. Marker

My friends 1994 automatic just started to stall without any warning. The stall is like you turned off the key. No bucking or reduced power like fuel starvation or the like. No rhyme or reason that is consistent. Speeds can vary from sitting at stop sign to doing 60 on the freeway. Flat, corners, uphill and down. Temperature not an issue, the fans were re-programed 2 years ago and seldom gets above 204. Engine can be just starting to warm up driving only two blocks to running at 60 MPH for 30 minutes. Opti was replaced last year with AC Delco unit.

I have replaced the ignition control module and the coil. Unplugged all electrical connections to Opti, ICM and coil and sprayed with electrical contact cleaner. Going to replace the crank position sensor next...just to throw parts at it.

Every time this has stalled, the engine starts back up without an issue. Might run 8 blocks or 8 miles or more without a problem. There has NEVER been a code recorded!

Forum research (and other non-Corvette forums) into the Opti and other parts indicates codes recorded, no restart conditions...extremely rough running. None of which are being seen with this car.

Any one out there that has seen this set of symptoms before?

antfarmer2

A bad ground on the icm or the thermal grease not right. Was dielectric grease used where needed?

antfarmer2

Make sure all plug wires are tight and clicked twice

John A. Marker

Dielectric grease was not used, it calls for heat sink paste to draw the heat away from the ICM. Searched all over for it and finally found it at Grainger. I removed the aluminum heat sink and clean it so there is no build up of dirt that would allow heat to build up in the sink. Also added non-heat conductive 1/4 spacers behind the bracket that holds heat sink, coil and ICM. So heat is not being transferred from head to heat sink and passed on to the ICM and coil.

All wire plugs to Opti, ICM and coil are cleaned and tight. Ground is good.

antfarmer2

The icm calls for heat sink grease the plug wires call for dielectric grease none was used there?

I might be wrong but I thought the grease was to transfer the heat from the icm to the heads to keep temp evan if so would'nt the 1/4 non conductive spacers over heat the icm?

John A. Marker

The heat sink paste is to help transfer heat from the ICM to the aluminum heat sink. The heat sink is larger than the ICM and has several fins at the top to help dissipate the heat from the ICM. The sink is made from aluminum because it transfers heat faster than steel and most other materials. There is paste between the ICM and the sink and the sink and the bracket that holds it to the head. There is still a small space between the head and the bracket. If the intent was to transfer the heat between the head, bracket, sink and ICM then the head would have been machined flat and the bracket would be sitting flush with the head. There would also have been the heat sink paste between the head and the bracket....but none. By spacing the bracket 1/4 inch from the head there is less heat from the head that gets transfers over to the bracket. It allows for more air to pass between the bracket and the head.

There are several posts in other non-Corvette forums about spacing the bracket away from the head. Several have measured the temperature of the heat sink before the modification and after. The conclusion was that the temperature was reduced at the heat sink by adding spacers. The other forums were using steel washers to space the bracket away from the head. I figured that the washers would still transfer heat. The material I used is good for up to 600 degrees. Your not trying to keep the temperature at the head and the heat sink the same. You want the ICM cooler.

antfarmer2

Then my mistake I thought the lt1 coolant system went to the heads first to cool them and that was why the icm was bolted to it to conduct the heat to keep it evan if so the non conductive spacers would mess this up. I learn something new every day. Still I wonder why they would bolt it to the head and not away from the engine if that was a problem. When I did mine I put heat sink on the head too.

kimmer

It does cool the heads first and your observation makes sense but it seems John's friend has the same problem either way. P.S. I put compound on the head also.

antfarmer2

Maybe just a bad main ground but sure he checked that. Did you try smacking the computer to see if it dies?

John A. Marker

Well the perfect answer seemed to be the crank position sensor! Research on this sensor had all the symptoms that are being displayed. But I know now that I can rule this OUT as the culprit.........GM put it in the LT5 but not the LT1 until 1996. So I can't change out something that is not there.......DAMN and that was looking so good as the solution!

antfarmer2

antfarmer2

I am not kiddig it is a cheap test.

N42375

Antfarmer2 is correct on smacking the computer. My mechanic calls it doing a FONZI. My 90 coupe had an issue last year when it would would run great for 400 miles,then wouldn't go 3 blocks before missing and then dying. After you give the computer a good rap,one of two things should happen-it's either going to get a lot better,or it wouldn't run at all. I got lucky, mine went from not making it to the end of the block to running great for the 40 miles needed to get to the shop to have the module replaced. The clue that I had that the computer was going bad were codes of high voltage on 3 items one day,then low voltage codes on the same items the next day. The clincher was getting a MAF code when my car has a MAP,not a MAF.

1993C4LT1

Yes, I too would hit the computer while engine is idiling. Not trying to annoy, just trying to help

rickcarol

I had a similar problem recently and finally tracked it down to the TPS (Throttle Position Sensor). No codes, died under inconsistent conditions, etc. Good luck!!

John A. Marker

Haven't been able to get back to her car for a couple of days. Looking toward fuel issue at the moment. Will attach the FP gauge and tape to the windshield and see what we see. So far nothing has worked as a solution. Still no codes being recorded.

rickcarol - how did you determine that the TPS was the issue? Can you provide a clue??

antfarmer2

Be carefull closeing the hood some have cracked there windshield.

kg4fku

Does the dash and other associated lights and indicators remain on as the engine stalls? Not after the engine dies, as you have told us the car restarts immediately, but just as the engine stalls.

I have a hunch you are looking at a computer issue as everything else should or would throw a CEL.

You said there has never been a code recorded. Are you basing this on actually reading the codes on the car or just off the lack of CEL being lit? Have you tried reading the computer for any stored faults that may or may not have set the CEL?

On a secondary train of thought... are you sure the CEL bulb is actually working?

John A. Marker

All other power to EVERYTHING remains on when the car stalls. Everything is good from radio, lights etc.....car just stalls.

And I am pulling the codes by connecting #4 and #12 (remember this is a 94) and reading any codes on the dash. NO Historical codes and nothing else coming up. The Check Engine Light (CEL) is working. Actually reading the codes not looking for CEL alone.

Research has indicated that fuel pump would not throw a code. Someone on another Forum had the stalling issues with no codes and it turned out to be the fuel pump. Not going to just throw parts at it, which is why we are putting the gauge on and seeing what happens. FP should be at about 40 with everything connected.

kg4fku

I realize it is OBD-1. Wish my 96 was too.

Anyway. Reason I asked about the power is that bit of information rules out a complete electrical failure like a bad battery, alternator or main ground.

Since the OBD-1 system doesn't have the CPS sensor how does the system control fuel injector timing and duration?

Sounds to me either the fuel injectors are shutting off or you are losing ignition control. Since you have already replaced the ICM then there are only two things I can think off, computer or whatever controls the fuel injection system.