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92 f150 300 6cyl. runs very rough,will not idle until warms up

Home Forums Stay Dirty Lounge Service and Repair Questions Answered Here 92 f150 300 6cyl. runs very rough,will not idle until warms up

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This topic contains 3 replies, has 3 voices, and was last updated by Jeffrey Voight Jeffrey Voight 3 weeks, 4 days ago.

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  • #963408
    russell fishback
    russell fishback
    Participant

    Had this problem. replaced MAP sensor and it ran great for a few days and quit under power. pounded on the Map sensor and it ran great. next morning back to the same issue of extreme erratic idle. needing to fluctuate the throttle to keep it bucking and snorting. foot off throttle and it dies. If warmed up it may run properly or may not. I wonder if the MAP sensor could be bad even though new. is there an easy test for the functions of the MAP sensor. one other item confuses me. when the key is turned on the fuel pump engages but does not shut off. Both tanks behave this way.

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  • #963410
    Nightflyr *
    Richard Kirshy
    Participant

    I wonder if the MAP sensor could be bad even though new.
    Would be the first time a “new” part is defective, especially if it is aftermarket and not OEM

    I wonder if the MAP sensor could be bad even though new. is there an easy test for the functions of the MAP sensor.
    If you can view live data from a scan tool that should help.

    one other item confuses me. when the key is turned on the fuel pump engages but does not shut off. Both tanks behave this way.

    Has it always acted this way?
    Not sure about operation of your fuel delivery system, But if it can not retain proper fuel pressure, perhaps that is why your fuel pump is continually running.

    #963412
    russell fishback
    russell fishback
    Participant

    Yes I agree Bad MAP sensor probably… it was very cheap. The fuel pumps continuing to run is a new issue. started with this problem.

    #963417
    Jeffrey Voight
    Jeffrey Voight
    Participant

    Probably 2 seperate issues. The new map can certainly be bad, but bad wiring or connectors could also cause this. A scanner can monitor signal during a jiggle test to help diagnose this.
    The fuel pump could have a bad relay. Replace or swap the fuel pump relay with a known good one. If that doesn’t fix it, check the primary circuit at relay box. If you have voltage continually on the primary side the vehicle thinks it’s running or cranking. This can be caused by a bad or shorted ignition switch or PCM. Depending on the vehicle the ingnition module, crank or cam sensors will report that the engine is or is not running. The first thing to check though is the fuel pump relay itself.

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