jrappl

I have a 92 LT1 auto with 66K miles on it. The car looks well taken care of and looks stock. It's had this problem since I got it 8 months ago. I'm just getting time to work on it now.

When cold the car will start and run fine, sounds great, no missing lots of power. Once the car is driven for 10-15 minutes and everything gets up to temp it will start missing and within a few seconds will stall. While this is happening the tach needle will start to bounce from 0 to 6K. It will restart and run poorly for a few seconds and then stall again. After restarting a couple of times it will just crank without starting. If I let it sit for a few minutes when I try to start it it will fire (puff) one time and then just crank without starting. If I try to start it again without waiting I don't get a puff, it just cranks.

Sometimes when hot and trying to restart the tach needle bounces but it never does when the car is cold or running right before all the way warmed up.

The service engine light works (comes on at startup and shuts off) but is off.

I have 40psi fuel pressure at the fuel rail. There is no gas in the vacuum line for the fuel regulator.

I did a search online for "LT1 stalls when hot" and the majority of hits said it was an ICM problem. I looked at the ICM and coil and they were pretty dirt, I assumed they were original and 22 years old so I replaced them. I cleaned everything and used thermo compound. I used new AC-Delco parts.

I still have the problem.

Does this sound like an opti problem or an ECM problem? How can I check? I have a FSM pdf but I find it hard to follow.

Any help is appreciated.

TorchTarga94

Opti

jrappl

Can I test to be sure it is the opti before replacing it? Can you list the steps to do the test?

I did read an excellent post on replacing the opti on this forum with lots of great pictures. How long should I expect to spend on that job, never did it before.

Drew95

Check your PM...

Also, if it turns out you do need to replace the Opti, that's the perfect time to replace/upgrade your water pump if you had any designs of doing it anytime soon.

Hope it helps.

jrappl

I have a new piece of info. I pulled the ECM to check for corrosion, bend pins, or anything that didn't look right. Turns out I have a junk yard ECM!

Apparently it's from a 93, maybe somebody can read what else it says. I guess this problem was here all along even though the PO assured me the car was perfect and I could drive it anywhere...

The ECM number does look correct from looking up the part to purchase. The PROM says DELCO 5224 5224.

bogus

Are you getting any codes?

I doubt opti... I am thinking either ICM or CTS (the sensor on the waterpump). If you are getting an H15 code, then it's the CTS.

If the opti is dying due to heat, I would expect a code - H16/H36.

jrappl

It does not seem like an opti problem to me either as I have the bouncing tach needle and one other thing is that the first cooling fan seems to be coming on and turn right off when the car won't start.

No error codes. The check engine light is not on. It comes on for the startup test but then goes off. I just replaced the ICM and Coil with new AC Delco parts.

I took the ECM out of the car and removed all the screws but I couldn't pull the motherboard out to look at it. Is there a trick to getting it out or do I just pry it out at the side plate? (the one with the connectors)

TorchTarga94

Yes, most of the time no start heat related issues are related to the ICM or CTS in the w/p. However, you stated bouncing tach needle. That's a pretty positive sign of opti failure. Hopefully its not, but myself and many others have been in your shoes with the bouncing tach issue. Runs great when you first start up, drive down the road 10-15 minutes later you notice a slight miss then a hard miss with the tach bouncing around, then the engine dies and all it does is crank when you try tp restart. Wait 15-30 minutes and ur good. Then repeat...

Most likely the opti man. You can whip out the volt meter and fuel pressure gauge and trouble shoot for days but be prepared to replace the opti. First time give yourself a full day to get everything real clean and gather tools. Easy job. Real easy actually. LT1 is a fairly easy engine to wrench on. After replacing my cheap opti with a quality Delphi unit and doing a few friends cars I can swap an opti out in about 2 hours provided the water pump gaskets don't need to be scraped and cleaned off the block.

jrappl

I don't mind changing the opti if it fixes the problem. Are any of the after market ones any good, some say don't use anything but the AC Delco but they are not available new and the rebuilts are expensive. I see them from $70 up to $500 with the Delphi around $400.

Also, on another post it said to check the resistance of the injectors. 7 of mine are within specs between 12.3 and 12.5 ohms but the 8th only reads 11 ohms. Tested cold. Seems like that would be a different problem with maybe not running as smooth as it could. I do have 40 psi pressure in the rails with no leak down after 5 minutes. Should I swap it out? Do all 8 at the same time?

redcorvettedave

Went thru this recently it took me 6 months to get car right.

I replaced opti,water pump, coil wire and something i believe was the hidden problem that compounded every other issue.

What is oil temp?
check the oil sending unit and connecting pig tail...not oil pressure sensor but sending unit ..

jrappl

Very frustrating. I've read thread after thread and everybody has a different opinion as to the problem. Even in this thread one says "I doubt opti" and another says
pretty positive sign of opti failure"

I appreciate the help just would like a way to test to be sure what the problem is. I did get one post with some testing step and I'm working through those.

Tom400CFI

That's b/c people are WAG'ing. Follow a diagnostic process to PROVE IT.


Opti. THe tach gets it's signal from the primary side of the opti. If it's going wacky when it gets hot....it's very strong evidence that it's the Opti. I had the same problem and I PROVED it HERE <-Video inside.

Tom400CFI

&All symptoms of a bad Opti when hot. Watch the video that I posted in the link above.

LannyL81

Everyone is quick to say the Opti is failing at hot...with good reason, but not always the problem.

My '95 had similar symptons which turned-out to be the ECM.

If the ECM is located under the hood, then try this quick test:
Remove ECM from mounting bracket with all electrical connectors mated and lay it at the base of the windshield.

Get a wet towel and wrap it around the ECM.

Start engine and let it idle and see if the problem returns. Keep towel damp so that it keeps the ECM cool.

If problem does not return, then problem found, replace ECM. If problem does return, likely it is the Opti.

If the ECM is located under the dash instead, then you could also try the wet towel test. I just have no experience with the dash ECMs.

Let us know what you find.

bogus

I have issue with heat related failure of the opti... tach gets its feed from the coil, not the opti... so if anything, a bad tach filter would cause that bounce.

Also, as these cars age, the harness to the coil and ICM become suspect.

The opti side is a durable part, unless this is some unknown knockoff, the OEM opti pickup is good for a long time.

And why fail only at op temp? Why not wait until 180? There isn't enough heat soak this early in the run process.

Call me skeptical... but I have been around the LT1 for over 14 years now, and I have never heard of heat effecting the opti.

Tom400CFI

That is wrong. The signal originates at the Opti. The coil, does in NO WAY, generate a signal. The coil gets charged with voltage, that voltage is cut (by the ICM) and it crates a spark in the secondary winding. No way, the coil can "create a feed" for the tach. Not how it works. As for heat related...did you watch the vid I just posted??



Did you watch the video I posted?


Did you watch the vide that I posted?

Tom400CFI

Maybe I need to make this easier for some people....

Tom400CFI

The ECM circuits can still get hot inside the box, but still not a bad test. You could shoot a heat gun at the ECM when the engine is cold, to test the ECM, or get the engine up to temp, where the symptom happens, then throw the ECM in the freezer for 10 minutes, then test again.

jrappl

I'm ready to replace the opti and water pump and here's why...

I have already tested and have good fuel pressure always.
I have replaced the ICM and Coil with new AC Delco parts.
All exposed wiring and connectors look good, not brittle, no cracks, etc.

Starting with a cold car sitting overnight in a 80 degree barn:

I tested my injector resistance, 7 are within spec at 12.3 to 12.5 Ohms, one was at 11 Ohms. I tested and had a steady pulse at each injector.

Car started right up and I let it just idle. I set the dash to show coolant temp. The engine ran nice and smooth and the temps went up slowly (10-15 minutes) to 228. At that point the left side fan came on as expected. A minute or two later the engine speed became unstable, increased to about 1200RPM, then dropped, and cycled doing this 3 or 4 times then died. It restarted and ran for 10 seconds and surged and died. It would then just start and die immediately.

I put the test light back on the injectors. No pulse when the engine is just cranking, then an unstable pulse and it would start and then die.

Tested resistance of all injectors again (hot) looking for one shorting out but again, 7 are in spec though slightly higher than cold (12.7 to 12.9 Ohms) and the one out of spec actually dropped to 10.3, Not a short though.

I unplugged the out of spec injector and the engine still would not run.

Here is a key. While spending all this time under the hood with a running car I noticed an anti-freeze smell. It was very slight and my brother did not smell it until I made him put his nose down near the water pump.

I'm still concerned about the ECM so before I order anything I will see if cooling it makes any difference - it is a junkyard replacement...

Tom400CFI

Any update or further testing gone on here?