'03 Express Idle Problem

[Wesman]

Vehicle in question is a 2003 Express 3500 with the 6.0 and cable controlled throttle.

Approximately 2 weeks ago, while driving, vehicle started idling randomly between normal idle and 3,000RPM. It would stay at 3,000RPM, return to idle, and then go back to 3,000RPM. It was all but undriveable.

Brought vehicle into shop, checked for vacuum leaks, sticking throttle, dirty throttle body, pirated air, damaged wiring, ect. No issues found. Replaced IAC valve, vehicle idled normally and stayed that way for the past 2 weeks without issue.

2 Days ago problem mysteriously returned. Vehicle will not idle below 2700RPM, even with A/C on. Disconnected battery, replaced IAC valve again, took vehicle to dealer to get reflashed, still will not idle. Used Genesis scan tool to command idle at 600RPM. No change in idle. PCM reports correct RPM, Throttle position, Coolant temp, IAT, and IAC counts of 148-156.

If I manually adjust the IAC pintle and leave it unplugged from the harness, the engine will idle perfectly at the set RPM. It also drives fine when the IAC is manually adjusted.

Once the IAC is plugged back in, the PCM immediately retracts the pintle completely, allowing for excessive bypass air and high idle of 2700-3000RPM.

This issue has more than a few technicians stumped, including myself. Any suggestions or input is appreciated!!

[Cutlass]

I seem to remember a car I had once that had a slightly skewed high TP sensor at idle. It caused the IAC counts to climb up like it was in throttle cracker/follower mode. Of course it only idled up to about 1600 rpm but it could be similar to your problem.

one question, when you commanded the IAC to 600rpm, did the IAC move but the idle didn't change? Or did the IAC not move at all?

[Wesman]

I seem to remember a car I had once that had a slightly skewed high TP sensor at idle. It caused the IAC counts to climb up like it was in throttle cracker/follower mode. Of course it only idled up to about 1600 rpm but it could be similar to your problem.

one question, when you commanded the IAC to 600rpm, did the IAC move but the idle didn't change? Or did the IAC not move at all?

Sorry for the delayed response...

When the idle was commanded to 600RPM, the IAC did not move at all. It would retract, but would not extend to lower the idle at all.

Upon further diagnosis, we checked for power in circuit A and B of the IAC pigtail. It had power and ground in circuit A, but no ground in circuit B. So we pulled the computer, got the wiring diagrams, and probed the PCM wiring. Both circuits had power and ground coming from the PCM. From there, we tested for power/ground in the wiring behind the pigtail - both circuits were good, so we ordered a new pigtail.

The new pigtail came in today, so I will let you know how it goes.

Thanks for the help!!