My motor is a stock 2001 6.2L MX MPI I just got it back from the shop two weeks ago after having the port side head rebuilt and as soon as I started it I heard a pop and it will not idle below 1500 RPM now. I brought it back to the shop last friday and we swapped out the ECM and the TPS with fresh ones to test it with no changes in the idle. He thought he ordered the wrong IAC so I just got it from him today and put it in and it still has no change. Are there any tricks with these motors I am stumped at what could be causing it to not want to idle right. I have gone over every wire and I am replacing a few grounds that look a little corroded but it all looks to be the way it should.

MAybe a vacuum leak at the intake sucking air causing it to idle high just a thought. Hope this helps.

Hmm I didn't know the motor had any vacuum lines there is one hose coming off the throttle body that goes to the bottom of the motor by the mounts but it goes behind a plastic cover I can't see how that would affect it. One thing that is strange is pulling the IAC and TPS while the motor is running makes no difference. Only the MAP sensor when I pull it out it will idle down for a second then shoot back up.

When I start the motor it idles fine for 10 seconds then it slowly creeps up to almost 2,000.

I know on a motor vehicle with IAC that there is a procedure to recalibrate after replacement. I found some threads while looking for an answer for my buddy that had this similar type problem with an early 2000's Maxima. I know marine application a lot different but I bet there is something you need to do to adust your idle. Also, any codes from computer???

No when we tested initially it threw the IAC code so we replaced it but no new codes. I have read in other forums that it could be stuck in service mode, also one guy had his TPS and fuel pressure wires switched and that caused his to idle high like mine.

The shop said the test they ran on the ECM which was throwing in a 350 mag ECM might not have been accurate after all since I have an early model 6.2. More testing to come.

The pop you heard might have been a backfire. I have seen backfires on throttle body efi engines bend the throttle plate to the point its open enough to let in enough air to get a high idle but the TPS still shows a mostly closed throttle postion relative to the throttle shaft and not the blade itself.
Might be worth checking.

Best Regards,
Ray @ Raylar

That was one of the first things I checked was the throttle position and the plate it is fully seated and not bent. When I put my thumb over the IAC hole above the plate with the IAC installed and plugged in I could make it idle at the correct RPM by regulating the amount of air it was taking in myself and it was not getting air from anywhere else. The IAC is working fine it would seem that either the computer is telling it to open or another sensor is. The only sensor I havn't checked is the MAP. The shop has it now he has a Merc rep coming to run tests and have the engineers figure out what is going on.

turned out to be a bad computer. I had it reprogrammed all is good.