I have a 1988 GT auto and recently I've had some strange behaviour.

The rpm needle jumps around when I let off the gas. (Just sometimes, not always)
Sometimes when this happens the engine stalls completely and it's hard to start again.
Then suddenly the engine pops on and runs perfect again.

Can it be signs of a failing ICM ?

pull tach filter then try again
See my Cave, HE Ignition

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Ok, but does a faulty tach filter really make the engine stop and die?
I have read that a lot of people even remove the tach filter and continue to drive, this made me more suspicious about the ICM.

quote Originally posted by theogre: pull tach filter then try again See my Cave, HE Ignition

quote Originally posted by kjelle69: Ok, but does a faulty tach filter really make the engine stop and die?

That and worse can kill ICM, HEI Coil, etc because when the tack parts "die" can causing them to overheat and kill them too.

Ignition doesn't care if tach and filter are not connected but People bypass the filter so OE tach still works can have same problem w/ iffy tach or wiring to the tach. Most Aftermarket Tach can have same problems too.

So just remove the filter and check again then leave filter off trying to fix what else maybe wrong because tach parts may still be bad.
When you fix whatever then try using tach again.

Take a close look at the connections on the ICM for corrosion and tightness.

Similar happened to me not that long ago. Wound up being the connections at the coil (V6). The little safety tab broke allowing the connector to back out a little. Engine would pop-fart a little, tach jumped around a little. Engine stalled a few times, hard to start etc.

Temp fix was a zip tie to keep the connector seated.

Guys, I think I found the problem.
Look at the rotors, the spark gap was way to big on the old one. When I changed distrubutor and rotor to New MSD's from Fiero store the car seems to run just fine again :-)

Ok... Update again.
Had to push the car a couple of blocks to get it home after work since it stalled again after a series of jumps of the tach.
After this it was completely dead. No spark at all.

When we finally got it home we changed the ICM and directly after this the car started up fine again. Soo. I really hope that the ICM was the final solution to this frustrating problem. :-)

Fiero having a blast in the snow :-)