350 MPI Throttle Plate Question

Discussion in 'Mechanic's Corner' started by jkhoch, May 3, 2021 at 4:04 PM.

  1. jkhoch

    jkhoch Member

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    Good afternoon. I'm wondering if my throttle plate is the issue of a rough idle I'm having with my Mercruiser 350 MPI. Should the throttle plate be completely closed at idle? I have a slight gap when at idle. I assumed that the IAC handles all idle air flow. I've checked vacuum, all sensors, fuel, wiring harness and can't seem to chase it down. I'm down to either this or the ECM I think. Thanks!
     
  2. tuner

    tuner Well-Known Member

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    The throttle body provides the minimum airflow requirement, the IAC adds extra flow for cold starts, gear engagement, and idle stability. If the blade is closed too far, the engine may have stalling issues and unstable idle. If the blade is open too far, the idle will hang due to excess airflow. Neither condition will cause a rough idle, unless the speed is below specification.

    To complicate this issue, TPS voltage determines commanded idle speed, so it is important to have the correct blade opening and TPS voltage at closed throttle. You will need a MEFI scan tool for proper adjustment. Scanner data will show the TPS voltage/angle and the IAC percentage at idle.
     
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  3. Pequod

    Pequod Well-Known Member

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    Check the Hall effect sensor in the distributor cap.
     
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  4. crhodes

    crhodes Well-Known Member Supporting Captain

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    See you are in California. Not sure if you can get non-ethanol gas. When I could only get ethanol gas, and when out of gear, my 351 Ford MPI (purchased new from PCM in 2006) would hunt like a diesel with oscillations increasing in amplitude until she died. In gear it was a rough idle. Threw parts at it with no success. Our marina started carrying non-ethanol again and literally within 15 minutes she started running great and haven't had an issue since. You have a GM 350 but worth thinking about before throwing pars at her.
     

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