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Thread: GM IAC issues

  1. #1
    Carb and Points!
    Join Date
    Aug 2016

    GM IAC issues

    Hey All,

    I am having issues with a Mercruiser 350 MAG MPI that has an IAC that is not responding. Here is the background on it. We bought the boat last year at the end of the season and I noticed that it had an occasional hard starting condition where it would start but idle low and stumble until you bumped the throttle. This season the condition continued and I started to investigate the issue. I noticed that the engine does not throttle up when cold and then return to idle after it warms up. Physically the engine is identical to the 5.7L L31 R code that is in the 2002 GMC Express 2500 vans. The only difference is the ecm being an MEFI 3 and the wiring harness. I removed the original IAC and cleaned the passage on both sides of the butterfly and everything is clean. The IAC appeared to be stuck at the pintle so I ordered a replacement IAC. I installed the new IAC, cycled the key switch for 10 secs. a couple of times to seat the pintle and then started the engine and the condition remained. I located a copy of Scannerpro for the engine and all four readings related to the IAC show the IAC steps when cold (70 deg) at 125 then slowly returning to around 50 when warm. This appears to be within the specs for the ECM. Next I purchased a noid light kit that has a light specifically for the GM IAC. I hooked up the noid light and turn the key on/engine off and the red and green lights flicker briefly indicating they are receiving the command to retract to increase idle for startup. The lights both then stay solid red and green. When I turn the key off the lights flicker again indicating they are receiving the command to extend for shut down. I am at a loss as to where to proceed next with this. Any help is appreciated.

  2. #2
    Fuel Injected!
    Join Date
    May 2016
    Sounds like maybe a glitch in the ecm. Do you have access to another ecm?

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