94 Z28 LT1 shutting down randomly after running for a while

**LDM** · 10-21-2025, 11:53 AM

Definitely got a weird one here, looking for anyone with some advice, especially since I haven't had the time to really properly spend the time to fully diagnosis this like I've needed to.

Ok, 2 winters ago I needed to replace the optispark as it was 20+ years old and failing. Went with a Petris optispark for the 94 Camaro, also swapped out the plugs, wires, and coil while I was at it as all that equipment was also around 20 years old. I also run an MSD digital 6 with this setup. Last year after getting her out from winter storage, ran all year without issue. Put her away for last winter, did nothing but let her sit like usual and pulled her out on April 1st. Sitting in the driveway idling while I moved a few other cars around, come back after maybe 10-15 minutes, engine is not running. Go to start, cranks but no start.

Ok, maybe something failed over the winter. Old modded car, happens. Start doing some basic troubleshooting, verify fuel pressure, verify spark, still no start. Swap out the ICM as these are known to fail without warning, had it happen before and I always keep a spare on hand. Still won't start. Checking for any loose plugs, find nothing. Finally, as a rare last issue as PCM failure don't happen much, swap out with the extra PCM I keep on hand in case one decides to brick while programming a new tune. Go to start the car, immediately fires up. I take her on a quick spin around the block, no issues, park her back in the driveway. Had a few other things I needed to do that day so I leave her sit and figure I will take her out on a longer drive now that I know the PCM is the issue.

Or so I thought. Fast forward a week or so later, start her again to take her out on a drive. Idling in the driveway for maybe 5-7 minutes, dies again. Try to start again, cranks but no start. Really? Pulled the PCM again as this was an older reman unit that I happened to have gotten as a backup. Don't see any issues, put it back in, go to start and starts up just fine. Dies again a few minutes later, won't restart.

Since I know this PCM is a reman and I can't remember about the other one, I decide to get a working old PCM out of a junkyard. Get the PCM, load my current tune, start the car, no issues. I let her idle for a while to see what happens, maybe 15 minutes later, dies again. Can't restart. Pull the PCM out, check everything again, put it back in. Starts right up, idles for a while, then just dies again, won't start up.

Pull in the PCM and putting any of the 3 I have in seems to allow it to start again but eventually will die after idling for 5-20 minutes. I have my computer hooked up datalogging, start comparing info, find that the alternator is running a bit low at 13.4-13.5. Ok, maybe the alternator is not charging up the battery fully and I've been chasing my tail on the wrong thing. Get a new alternator and battery, swap those in.

Car fires up, I let her idle again for a while to see what happens. About 20 minutes go by, 14+ charging from the new alternator the entire time and idling without issue. Just about ready to hop in a take her for a test drive, dies again. Won't start. Pull the PCM, put it back in, starts up. Idles for around 5 minutes, dies again.

Datalogging info doesn't show any issues, when the engine dies it just drops the RPMs to 0 and the MAF to max like you would expect. Injectors are all firing normally, no knock retard, no weird MAF signals, nothing out of the ordinary.

This is probably one of the most bizarre issues I've had with any vehicle in a while. Its almost Nov 1st when she goes away for the winter and I didn't get to drive all season due to this issue. If anyone has any ideas I'm opening to taking a look when I hopefully get a chance to get out in the garage and spend some more detailed time diagnosing this problem.

**JOATMON** · 10-25-2025, 11:14 AM

Sure sounds heat related. I’d be suspect of the ignition coil and the ICM. Make sure the ICM has heat sink paste on the flat surface and consider the spacer trick to keep head heat off of it.

**LDM** · 10-25-2025, 05:08 PM

Originally Posted by JOATMON

Sure sounds heat related. I’d be suspect of the ignition coil and the ICM. Make sure the ICM has heat sink paste on the flat surface and consider the spacer trick to keep head heat off of it.

I'd tend to agree but my ICM is moved and mounted on the flat area above the fuse box and has heavy duty thermal paste applied to keep it cool. I also have a spare ICM that I keep around since they are known to just fail at random and for no reason. Swapping in the spare does nothing to get the engine started again. Ignition coil is brand new, I replaced it when I replaced the opti.

I'm almost leaning toward the idea that that MSD digital 6 might be the issue. It is over 20 years old. I need to dig up the diagnostics for it and see if there is anything I can do in regard to troubleshooting it. Maybe bypass it entirely and see if the engine will start and run without issue.

The weirdest part is that unplugging the PCM will allow the car to start and run for a while again. It seems to be resetting something when I unplug it but I can't figure out what. Manually clearing the LTFTs with a data logger doesn't do anything and its not like the car is running long enough to really store any data that would effect running. Also, start is always done in open loop so its not like the closed loop trims would have any starting effect.

I was able to get her back in the garage the other day, she ran for around 10 minutes and then shut down again. Won't start back up again. At this point I'm hoping to find some time next week to take a deeper look into this issue.

**JOATMON** · 10-25-2025, 07:36 PM

Sounds like a strange issue. Don’t rule out the coil shorting out when hot just because it’s new. The only time I can ever had a a no start condition on mine was a bad MSD coil. Maybe check the injectors impedance individually to make sure a bad one might be getting hot and pulling the circuit voltage down. Check all injector wiring for frays or cuts. Check all grounds, particularly the one you messed with when you relocated the ignition coil and ICM. Good luck and happy troubleshooting. Let us know what you find.

***SSlowBoat*** · Yesterday, 08:08 PM

had a similar issue with the bs petris optispark thats SUPPOSED to be special, well its the same as the local parts store ones. would heat soak and randomly die, long crank, just not start. swapped optis with a parts store one and it stopped.

i have had a msd digital 6 box die too. similiar issues. box was getting searing hot after 10 mins.

id remove the msd box anyway, they kill optis.

**JOATMON** · Today, 10:07 AM

Shaun’s comment jogged my memory about my wife’s ‘94 Z28. It had three optispark distributors in ~130k miles. When the first one failed about 50k miles, it would crank and run, then die and not restart until it set for a little while. Luckily it was in the driveway. Replaced with a stock AC-DELCO that failed at about 100k, dead had to get towed in. Replaced it with a MSD spline drive with the venting update, it was still working great when she sold it around 130k to buy a new truck. So long story short, Shaun has likely nailed your problem.