What's a bad ICM and/or coil feel like?
 

Long story short - 1996 Z28 Convertible, M6, 89k miles. I had a low rpm, high load mis-fire that would clear up over 2000rpm, and some breaking up over 4500rpms. Replaced the opti, wires, and plugs. Completely eliminated the low rpm, high load miss, you can stand on it at 1200rpm in 6th and it just pulls smooth and clean. Top end break up also largely diminished. What I still get are like flat spots above 4500rpm. They are not consistent to a specific rpm, and the only occur about 50% of the time, half the time the car pull cleanly to the shift point. Before, the car would misfire, you could audibly hear it in the exhaust, a soft pop. Now, you'll feel the car very briefly just fall off - it doesn't stop pulling or misfire that I can hear, but it softens up, and then its right back before you know it. Another important component, this is much more likely when the car is warm. No other symptoms, car runs excellent, gets great mileage, and is still otherwise making all the power it should.

So, I'm going to do some fuel pressure testing, but I checked before I did the opti and was in good shape, so I'm thinking this is still the ignition. I still have the stock ICM and coil and I've heard you can get some funkiness with the ICM as it ages and is subjected to heat - the fact that it happens with heat give further credibility that it may be electronics of some sort. I considered knock retard - but the recovery is far too fast.

Those of you that have had a coil or ICM start to fail - what symptoms have you had? If you've got other suggestions, I'd be open to them.

I was having a lot of issues with mine cutting out at higher RPM also. Up until about 4500 - 5000 the car ran perfect but after that acceleration was very choppy and there was similiar backfire noise as you were describing.

When I scanned the car it said I had a bad coil. So I changed that and it helped but there was still a miss at high RPM. I ended up swapping ICM's with a buddy(94' formula) and when I took mine out for a spin it ran perfect.

My suggestion would be find a buddy who will let you try his ICM and see if that helps. Your issues seem very similar to what my car was going through.

Good luck man and keep us posted!

That's what I'm gonna try, I have a fellow LS1tech-er up in Rochester that has already agreed to let me try a spare ICM/coil he's got hanging around, be nice if it was that easy.