redzr1

I am starting a new thread on this issue I am trying to resolve as the symptom I am not able to get resolved is primarily the engine going into a rev limit rather than a code 36 continually being set as listed in the following thread.

http://forums.corvetteforum.com/c4-t...post1578426287

I have a 92 Corvette (~52k miles with automatic transmission) that cuts out at ~3500 RPM in a manner that is like a rev limiter kicking in. This was occuring at the time I purchased the car a short while ago and at that time it would after a few events of hitting this level and cutting out set the check engine light with a code 36. Based on the code 36 being present along with a leak from the water pump seen, I replaced both the optispark and the water pump. Initially after this replacement a code 36 would set and I could run the engine up to 5500 RPM. After disconnecting the battery a couple times to clear the code 36 in memory the car will now go into 'rev limiting' at around 3500 RPM under every attempt, but will no longer set a code 36.

Anyone understand why the engine would limit itself in this manner and not set a diagnostic code? I have looked through the driveability flowcharts in the factory shop manual and cannot find anything that addresses this type of issue.

redzr1

Wanted to make an update for anyone that looks at this thread while searching to repair their car. After replacing the optispark for the second time and not seeing the issue go away, I then put in new AC Delco Spark Plugs and wires. This did not solve the issue, so I borrowed another ECM (GM#16159278) and put it into the car with the PROM out of my original ECM. The rev limiting at 3500-4000 RPM went away with the ECM change and then came back when I put the original ECM back in.

I am now looking for a good replacement ECM and also will be following up with our service engineers to figure out why the test for remanufactured units does not catch the problem seen with the one in my car. Thanks to all for their input. Unfortunately I still have to pull the water pump and reinstall as it is leaking a little bit after putting the last optispark into the car.

jaa1992

Good farkin luck finding a rebuilder that actually vibration and heat tests the ECM. And the 278 ECM is very difficult to find. IIRC I payed just over $200 for a very used known good ECM. Now I have a spare on the shelf for everytime the FSM points to the ECM I can swap the spare in and check again. Its saved me a few times.

A couple more things to try if your issue comes back:
Replace the harness from the opti to the ECM connector on the passenger side of the intake. It gets funky and sometimes has intermittent issues.
Keep an ICM and coil on hand for a quick swap. Sometimes the ICM is perfectly happy until it gets real hot, same with the coil.

redzr1

I did not indicate that I had also tried out changing the ICM and coil both with neither one showing any improvement prior to either heating up. I tried checking both of those before I did the second optispark replacement. There was definately an issue with the first optispark I put in as I had to crank 2-4 seconds most times to get the engine to fire up, so that lower end optispark that was not an AC Delco one had more issues interfacing to the ECM.

Once I get my hands on a good ECM, I will probably do the same as you and get a second good used one to put on the shelf for future diagnosis.

quickcat

I found a place online a few years ago that had them on the shelf. Good cardone units. It was weird cause I couldn't find one anywhere but this guy said he had a couple of them. I ate he core charge and kept my original as a spare cause it runs the engine fine.

It was autoparts.com or something like that.

Definately sounds like an ecm problem to me, although a weird one though.

Matt

PLRX

Fuel Pressure Regulator

Fuel Pump

I had that problem in my 96. The engine is slightly modified and it was dying after 5,500 RPMS. I installed a High-Flo pump and problem solved.

redzr1

Fuel pressure was one of the very early things I looked in determining the cause of failure. With the bad ECM when it would set a code 36, it would allow the engine to run very strong up to 5500+ RPM which also ruled out those two items even though I made sure to check them. The clear bad component was the ECM, but no clear way to have diagnosed it without a second one available.

PLRX

jaa1992

I know you had that in your original post. I was adding it for those guys in the future that just read this one. You did it right

We just have the redheaded step-children of the LTx world

jerhofer

I am having this same problem with my car. My engine is from a '94 Vette in a 1995 Porsche 968. I recently picked up an ECU from a '95 LT1. Will the '95 ECU work with the '94 motor?