I have a question related to the timing control module.
My '88 Fiero GT 3.4L died on the interstate, it turned out to be a bad ignition coil. Replaced it but the engine wasn't running good. Replaced the ignition timing module and the performance was back to normal. However, this seems to last for about 10-15 minutes and then it seems like I have a bad ignition module again, tried another module, same thing. Checked my base timing, took the average of cylinder 1 & 4, made a minor adjustment, still have this issue.

I noticed that at higher RPMs, 3k+, it runs smoother, as if the timing is stuck at some advance setting were the ECM is not controlling timing advance any more, is my guess.
When I look at the circuit diagram and since I do not have a code 42 I think I have reduced my check list to the following:
1) A ground problem?
2) Maybe the advanced signal between the ECM to the EST has an intermittent connection?
3) ECM issues?

Anything else I can add to this list?

Anyone ever experienced this problem?

Any suggestions on what could be causing this?

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Nick D.
'88 Fiero GT 5-spd
'03 Jetta GLS TDI 5-spd
'07 Toyota Sequoia Limited

ICM can "die" from:
Overheating for any reason. Example: Exhaust heating the dist, coil, etc, from missing shields or leaks.
Anything on High Volt side bad... plugs, plug wires, coil, etc.
Pick up coil has problem making ICM look "dead."
Iffy Ground(s) or other wiring. HEI ICM uses 1 of 2 mounting screws as ground. Other ICM "ground" from ECM isn't a real ground. See my Cave, HE Ignition, Ground "Myth" notes

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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

quote Originally posted by theogre: ICM can "die" from: Overheating for any reason. Example: Exhaust heating the dist, coil, etc, from missing shields or leaks. Anything on High Volt side bad... plugs, plug wires, coil, etc. Pick up coil has problem making ICM look "dead." Iffy Ground(s) or other wiring. HEI ICM uses 1 of 2 mounting screws as ground. Other ICM "ground" from ECM isn't a real ground. See my Cave, HE Ignition, Ground "Myth" notes

Thanks, I will look at your links tonight, the network where I work is blocking your links.

Btw, I did replace the pickup coil, cap and rotor.

Will be ordering new spark plugs and wires, but I shouldn't have to. All the ignition components have less than 10k miles on them.
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Nick D.
'88 Fiero GT 5-spd
'03 Jetta GLS TDI 5-spd
'07 Toyota Sequoia Limited

[This message has been edited by npdimonte (edited 01-27-2023).]

Hey theogre,

I have been pouring through your stuff and the ESTHI and ESTLO signals make sense now. I had trouble locating the schematics but found them at a different site, here's the link 1227170 schematics.

I read up on the HEI, good read. Question, when I rebuilt my 3.4L I put in a MSD coil, is this more trouble than its worth? Everything else with the ignition is normal.

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Nick D.
'88 Fiero GT 5-spd
'03 Jetta GLS TDI 5-spd
'07 Toyota Sequoia Limited

All ECM/PCM w/ HEI and DIS2 have same fake "ground." Maybe other ignition setups like many Ford ICMs have this too but hard to find real data for other cars.
Another is Tan "Ground" bolted to engine is return line for O2 sensor.

MSD Accell and other "hot"/"performance" coils are crap and often make problems.
MSD is China and several versions in just HEI coils.
Even years before that many are/were made in Mexico thanks to Clinton and NAFTA and had problem too.

Example: I put Accell Mexico Coils on DIS engine and had to mod the ICM just to install them.
Worse, drive ~ 10 miles and the Coils overheat and/or made ICM to overheat and died until cooled.
Between Accell crap support and warranty I warn everyone since Accell = Crap.
Many here alone have problems w/ MSD too and have same warning since.

That ignoring many MSD things can burn the dist cap center button and why they sell copper button.