Originally Posted by gbrtng (Post 54569)

My experience with a bad ECM: 2 or 3 unrelated error codes would set. As soon as the check engine lit, the engine would barely run or just stop and not restart. I could not isolate the problem to heat soak because it would happen at any outside and underhood
temperature. Replaced several ignition components with no change. Corey hooked up his Tech 1 and left baffled after an hour. I finally decided to change the ECM and yep, that was the problem. This was on Mary's 91 L98 which uses a common junk yard ECM - I was able to score one locally for $55 - which crosses to about 10 other GM vehicles.

Originally Posted by GregCrowe (Post 54581)

Jeff tends to think 75-80% chance it's a coil problem, 20-25% chance it's the ECM. Taking bets on what it is ;)

If it's either of the 2, I'll have it fixed in not too long.

Then I get to take the car up to the tag agency and explain to them why it hasn't been tagged in 3 years......... LOL.

Originally Posted by GregCrowe (Post 54581)

Jeff tends to think 75-80% chance it's a coil problem, 20-25% chance it's the ECM. Taking bets on what it is ;)

If it's either of the 2, I'll have it fixed in not too long.

Then I get to take the car up to the tag agency and explain to them why it hasn't been tagged in 3 years......... LOL.

Originally Posted by GregCrowe (Post 54373)

OK, in complete detail:

When the car is cold or even just warm and being driven for the 1st 15 minutes, no issues whatsoever, even with full throttle. After the car has been driven and has sat, heat synching the engine, the miss begins. Normal driving with light throttle, no issue. When a load is applied, the miss begins. The miss is very "electrical" feeling. It does not come and go under heavy fuel delivery situations. It's just a very quick electrical miss and shudder. The car never dies nor does it even come close to dying. Again, it's only under medium to heavy load. If you want to make it miss the absolute worst, let it heat synch and then bog it down in a high gear at low speed and floor the gas. Then the car will shudder and jerk. If you try to accelerate through the gears at full throttle, the car just plain misses like it has 2-3 bad plug wires arching.

Originally Posted by LT5-Lee (Post 54911)

Greg, I think I found my gremlin..=D>

and I can't believe what it turned out to be. I was wiggling everything while the engine was idling and for the hundredth time, I played with all the sensors and then the MAP sensor and the engine died? I swear I checked it out many times, but it seems the rubber accordion seal on the connector must have gotten twisted and was not quite snapped on tight. My scan tool showed the correct voltage readings, but I guess when the engine torqued just right it lost connection? :dontknow:

I can not believe it because it was one of the first things I suspected and always made sure it was tight, but I had to squeeze it so hard before I heard the plastic snap, I thought I was going to need some channel locks before I actually heard it snap on!!!

I maybe a little premature as I have only made a few hard runs for testing it out because of traffic, but knock on wood, I hope I got it!

After doing some data logging I probably have some other issues that made it worse, like a lazy LH o2 sensor with low cross counts (or a really slow scan tool "Auto X-ray"),but I am preying the MAP sensor connector was the major problem. My problem wasn't just heat related, but it chugged so bad it felt like it would throw you into the windshield just like you described so I thought I would just pass along my good fortune just in case?

Originally Posted by XfireZ51 (Post 54913)

"My scan tool showed the correct voltage readings, but I guess when the engine torqued just right it lost connection? :dontknow:"

If you had put the scan tool on and datalogged while driving, I'll bet you would have seen the MAP voltage drop under load. ;)