Need Help, ECT 3 Modules, no cure ?

[JTWard]

I'm having a problem with my 88 K5. It's got a 350 TBI and I ran into a "Service Engine Soon" situation. We have replaced the ignition module to no avail, skipped the next one and put an AC Delco and 13 months ago we put in a remanufactured PCM from Dorman ( Only Dorman AC Delco) has them. So I'm trying going back, I am putting a Stock coil and cap and wires back on. That way the only thing that isn't stock, an Edelbrock #3704 TBI intake manifold. But I can't see any cause with that. So I don't understand how the Code 42 keeps coming back. The wiring is fine, It's had a new EGR Valve and switch? I cannot find a cause and effect in the EST system . Need help.

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[1987 GMC Jimmy]

Not trying to grill you, but how are you determining that the wiring is okay? I’d backprobe EST control, bypass, and reference voltages at the ECM connector to see if something comes up fishy. Otherwise, you’ve touched everything in the circuit. ECM, distributor, and wiring. That’s it. Are there any issues present at this time?

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[chengny]

I'm having a problem with my 88 K5. It's got a 350 TBI and I ran into a "Service Engine Soon" situation. We have replaced the ignition module to no avail, skipped the next one and put an AC Delco and 13 months ago we put in a remanufactured PCM from Dorman ( Only Dorman AC Delco) has them. So I'm trying going back, I am putting a Stock coil and cap and wires back on. That way the only thing that isn't stock, an Edelbrock #3704 TBI intake manifold. But I can't see any cause with that. So I don't understand how the Code 42 keeps coming back. The wiring is fine, It's had a new EGR Valve and switch? I cannot find a cause and effect in the EST system . Need help.

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Read the diagnostic stuff below. You'll see that the most frequent cause to throw a 42 DTC is a short to ground - or an open - in either the 424 or 423 circuits. Ring those leads out for both conditions. The pin locations (at the ECM ends) are traced in red below. The circuits traced in yellow are involved with the EST system but they wont cause a 42.

The other two causes for a 42 that comes back repeatedly are a bad ICM (which you have already replaced) and...unfortunately, an ECM that is on it's way out.

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[JoeR Jr]

This is a long shot, but a simple check. In the early days of EST, the PCMs were pretty susceptible to outside noise. On occasion we would find a car or truck setting random Code 42s and it was because the 4 wire harness coming out of the distributor was run too close to the plug wires. It would usually happen right after someone tuned the vehicle up, and then ran the spark plug wires in a different position.
As a technician, the next thing I'd be doing would be to grab a lab scope. I'd want to see what the EST line and Bypass line were doing when you started the truck. This code sets fast when there is a problem. I'd want to know if the PCM is even attempting to go in to EST mode. Unfortunately not everyone keeps one of them hanging around.
Joe