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Thread: Rough idle when a/c is running

  1. #1
    Join Date
    Feb 2017
    Posts
    1

    Rough idle when a/c is running

    Engine runs rough when a/c is on. RPMs drop intermittently and car will stall if i dont give it some gas. It also happens with the heater but to a much lesser degree. Check engine light is not on. Going to change fuel filter as she also has difficulty starting when the engine is hot. I did ignition wires yesterday and checked the plugs. They seem fine and were done 6k ago with NGK iridiums. I always run high test gas but have only owned the car for 4 months and can not speak for previous owners.

    Wondering if anyone has experience with this issue. I can't find anything about it in the forums yet.

    1990 tbird SC. 86k miles on odometer

  2. #2
    Join Date
    May 2002
    Location
    Dallas, TX
    Posts
    5,641
    Quote Originally Posted by Dixon View Post
    Engine runs rough when a/c is on. RPMs drop intermittently and car will stall if i dont give it some gas. It also happens with the heater but to a much lesser degree. Check engine light is not on. Going to change fuel filter as she also has difficulty starting when the engine is hot. I did ignition wires yesterday and checked the plugs. They seem fine and were done 6k ago with NGK iridiums. I always run high test gas but have only owned the car for 4 months and can not speak for previous owners.

    Wondering if anyone has experience with this issue. I can't find anything about it in the forums yet.

    1990 tbird SC. 86k miles on odometer
    Check for collapsed engine mounts. This is a rather common problem. You can raise the car on jack stands, and probe around from underneath... check that the upper mount finger is not resting on the lower opening. Should be some space between it and the lower part of the bracket. Something like this [-] instead of this [_]. If you do a search in the forums about this issue, it should be explained.. since its not obvious why mounts might cause this problem.

    If your mounts are OK on both sides, move on and try pulling codes.

  3. #3
    Join Date
    Jun 2002
    Location
    Zionsville PA
    Posts
    445
    Do you have a vacuum leak in the controls? It's odd heat does it that's why I think you might have a bad diaphragm or hose leaking.

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