90 Buick Century idling rough

I have a 1990 Buick Century that's been idling rough lately. It has the 3.3 V6 engine. The problem appears sporadically, and is most felt when cruising at around 45 mph, with no gas. At times the SES light comes on
and stays on for a few seconds. The codes stored are 22 and 42. According to troublecodes.net: 22 - TPS voltage was under 0.24 volts for 4 seconds when ignition was on. 42 - The EST signal did not change when the ECM applied bypass voltage to the ignition module. Fuel pressure is where it should be. No apparent vacuum leaks. Spark plugs, wires and oxygen sensor were replaced less than a year ago. I'd appreciate a pointer in the right direction. On a car this old, I'd rather not use the trial-and-error method of replacing components until the problem is fixed ;) Thanks. Svilen
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Intermittents suck.
Start by checking the connections at the ECM.

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Steve Mackie wrote:

corrosion in the ecu plugs. The ecu is in the right kick panel.
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Paul wrote:

I checked the ECM plugs last night. What am I looking for there? I didn't see any corrosion or broken pins. One of those plugs felt like it was not fitting snugly. I unplugged all three and plugged them back in. Then I test drove the car, and it ran fine -- I couldn't reproduce the rough idle problem. Should I have applied some contact-cleaning spray?
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Contact cleaner, then dielectric grease.

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You may need a replacement ECM. My '90 Century with 3.3 L exhibited similar problems then failed completely one day, requiring ECM replacement. Also check ignition module. Recent change of spark plugs and/or wires creates stress on I.M., good luck.
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Svilen Stoicheff wrote:

If it continues to work that's great. But if not, pull the ecm back out, take it apart, and examine the pc board for corrosion, etc. Pull the prom, clean the contacts and put back in.
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