Hello All:

I've searched this forum and I have swapped a few key items but to no avail, now I'm reaching out for assistance in diagnosing this frustrating problem.
Your help is much appreciated.

It's a 93 LT1 with 130,000 miles. I've replaced the Water pump/Opti 4 years ago (about 18,000 miles ago).

Symptom:
Once the car reaches normal temperature and some times a few minutes after, the car shuts right off as if I just turned the key to the off position.
Now this has occurred during high acceleration, while waiting for a green light and while turning into my driveway.

Once this new wonderful feature occurs, the car cranks but does not run until it cools off again. Once cooled down, it starts right up and run's until it heats up again. Definitely a persistent issue.

Things I've performed to resolve this heat soak issue:

1) Replaced ICM & Ignition Coil (Delco ICM & MSD Coil)
2) Verified Injectors are within resistance range (checked it when the car was cold-12.5 ohms)
3) Replaced Fuel pump and fuel filter
4) Verified the fuel pressure is in the normal idle range and it maintains pressure when engine is turned off . (checked when car was cold)
5) Verified the two connections from the ICM to the Coil are functional.
6) Verified there are no DTC codes present using datamaster.
7) Cleaned throttle body & IAC
8) Verified the batt connections to the battery are fine.

I was able to capture and log the ECM events when this issue occurred. Based on my limited knowledge of interpreting the results, I did not see anything relevant. Granted that my fuel maps need some work which I'll get to once I reposition my O2 sensors ( I have long tube headers). My left and right BLM banks are inversely off (rich on one side and lean on the other)
Some of the stats, such as MAP, water & oil temps, voltage,TPS, advance and retard timing are all normal. I did notice the knock count value was initially set at 565 when I started the cold engine, I found this to be odd. It didn't increment during my run.

My thoughts are to eventually replace:
1) Alternator
2) ECM
3) Opti

I still need to verify if the far-end of the battery connection is on tight
and EGR valve is still functional.

Well, what's the best course of action to continue tackling this issue in a cost effective manor? Has anyone been able to resolve this similar issue?

Thank you in advance
Hank

 

Ttt

 

Take the ICM off and have it tested. Everything you are saying points to the ICM heating up and failing.

 

Did you put some fresh "thermal grease" on the back of your new ICM when you replaced it?

 

Sure did. I applied Silver Thermal Grease on both matting sides.
thx

 

Will do, The FSM also contains some testing procedures.
thx

 

If a chronic problem then something you can duplicate in your driveway right? If so hook up datamaster and log a normal startup all sensors etc. Heat it up until it fails/stalls/cuts out. Datalog an attempt to start do listen for the fuel pump as well. Put a known working timing light on a plug wire and see if you have spark. If so a lot of things can be eliminated. I hate to say it but a lot of delco stuff is made in China now and no better than an Autozone part.. Infant mortality is pretty common in electronics so don't rule it out if no spark.. If it cranks should not be VATS...
Dave

 

Thanks for the advice Dave. Using the timing light is a good idea. I do have some logs of the stall and the restart, but at the time I only focused on the results of the Datamaster logs which didn't provide a clue. I do need to take it to the next step like you said.

In preparation for the next steps, today I documented some base measurements of the input & output of the of the ICM when cold. I also made a by-pass wire/connector to disable the Tach filter ckt by inserting it into the new MSD coil connectors and noticed the NEW MSD coil terminals did not make a tight connection. What the f#$).

Since I still have the original Delco coil, I inserted the same by-pass wire/connector (same thickness as the coil terminals) into the Delco coil connector and it made a tight connection.

My goal it to return this out-of-spec MSD coil and get a Delco or another MSD coil that has a tight connector and test.

I'm speculating that my original issue was the ICM and now the loose coil connector is the culprit. Nevertheless, I'll prepare for the worse and confirm the issue is the ignition system. Based on the FSM, I should be able to determine which ignition component is the culprit, once the failure occurs.
I also noticed that the stall issue did not surface by running the car today in the garage. All the other times I was driving the car when it stalled.

 

Just a quick update:
I replaced the MSD Ignition Coil with one that has a tighter connection.

Tested the car again and sure enough it stalled again, it did start right back up then stalled. Stayed on for a few minutes then stalled. I was ready this time with a few tools and strategy.

The fuel gauge maintained fuel pressure off the rail during the stall and during restart attempt.
Ohm all the injectors again but this time with the car hot and all verified ok.
Spark was present on #2 & #4
Spark was present on the secondary ckt of the ignition coil (spark Wire to Opti)
when it stalled and when cranking.
I measured the input/output of the ICM and it checked ok

Everything seems to be pointing to the Opti or the ECM.
My next task is to replace that dreaded Opti.

Any suggestions on the Opti? New MSD or GM?
With the vents?

Thanks
Hank

 

I am having pretty much the exact same problem on my '93 and I can feel your pain, it is a very frustrating problem. Haven't been able to drive my Corvette in weeks and that really sucks. Replaced the ICM and coil. Fuel system was fine, verified there was no spark when in fail mode.

See the thread I started here for info on the same problem:
http://forums.corvetteforum.com/c4-t...ral-times.html

I'm pretty sure the opti is to blame on my problem, but it could also be the ECM (I hope not). Try this test(the FSM is where I got this test), you will need a digital voltmeter. Disconnect the ICM connector, and find pin "B". This is the ignition signal output from the opti, to the ECM and then to the ICM. Put the DVM in 'AC voltage' mode, hook the positive line of the DVM to pin "B" on the ICM and the black line of the DVM to any good ground. You will either have to have someone look at the meter while you crank, or do what I did and put the meter face down on the windsheild where you can see it when you crank. There needs to be 1 to 5 volts AC on this line when cranking. I found that when my car wouldn't start, there was no voltage on this line when cranking, but when the car would start, I would have voltage on this line. If you have no voltage at pin "B" on the ICM connector, then I would suspect your optispark. If you replace your opti, I would recommend that you do the vent mod described in the link I posted below to increase it's lifespan. Your problem is a little different in that you have spark at the coil when your car is in fail mode, but your opti may still be the problem. The only way to know is to replace it. Did you check spark at all the plugs? If you have an FSM, start with the "cranks but doesn't start chart" in book 2 and go from there.

I had replaced my opti only several thousand miles ago, but it had a lifetime warranty on it. I sent it in and they found that the optical portion of my opti was going bad. They replaced that for free, but they also said I had excessive spark deposits in my cap from being sealed too well. I had sealed both upper and lower cap with RTV, but the opti was not active vented. I bought a vented cap from them and am planning on converting my opti to the active vent system like the 95-96 corvettes have. I ordered the complete new vacuum harness from GMpartsdirect.com for a 96 corvette and and am going to do the mod described in Corvette Fever. Here is the link to that article:

http://www.corvettefever.com/howto/16758/index.html

I haven't gotten my replacement opti installed yet (I'm waiting for the vacuum harness, it had to be special ordered) but I'll let you know if the new opti cures my problem.

 

bdw18_123
Wow, we're in the same boat! I'm curious if the Opti resolves your problem. Let's us know the results.

I did measure the volts from pin B, and at the time it was within range @ 2.4 volts AC, but this darn issue is so inconsistent that I think I might have measured it when it would have fired up. Since I had to disconnect the ICM connector in order to measure it , I have no idea
what point it was in.

At one point I had an ever so slight back-fire, which supports my thoughts on a defective Opti.

So what are your thoughts on All-ignition.com? Looks like they sell OEM and some other stuff. (China ?)
I really want an Opti that's reliable because I am soooo not looking forward to replacing the Opti. Removing that darn pully is a pain.

You know, I also RTV/sealed the cap on this Opti a few years ago thinking it was the right thing to do should the water pump give way.
But it's not vented.

 

Just an update for all who reads this post.

I installed and updated the new Opti to support the venting feature that the earlier models have. Installed new spark plug wires and routed them correctly. Cleaned and put everything back together and presto!!!

It no longer stall's and it's so much smoother than before !! It's sooooo nice to have the vette back in action. I enjoy my Acura, but there is nothing like the Vette.

I really began to suspect the ECM, but it's just fine.

A 396 and 140,000 miles and it's still going strong!!!

 

good to hear.

 

Thank you

By the way BDW18_123 was a great help during the process.