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Weird RPM drop

Discussion in 'Diagnosis/Help' started by 86GTMonkey, May 27, 2008.

  1. 86GTMonkey

    86GTMonkey Well-Known Member

    Is it normal for the revs to drop when I apply the brakes while idling with the A/C on? It'll go from 800-900rpm to 500-600rpm and shake like coked up squirrel.
     
  2. ST165-2765

    ST165-2765 Well-Known Member Donated!

    It is normal for RPM to drop when you put the brakes on. The ECU detects the brake pedel being depressed and lowers the RPM's

    A 300 RPM drop is excessive I would have expected 100-150 RPM
     
  3. dan1_721

    dan1_721 Guest

    Does your A/C Idle up work?
     
  4. Klue

    Klue Well-Known Member Donated!

    when you press the brake pedal with engine on: the brake booster is operated by engine vacuum, when you depress the brake pedal the vacuum storage in the booster is depleted. There is a valve that opens to allow the engine vacuum to "fill" the booster again, the sudden opening of this valve creates a pressure drop in the intake stream causing the engine to stumble. Its normal to an extent. the problem would be amplified if you are running the A/C because you have even more load on the engine.

    The stumble in the engine is the IAC reacting to slowly to changing pressure's in the intake stream. The car is old so don't be surprised.
    Like dan said does your a/c idle up work? The engine should be at 1000-1200 rpm when A/C is on
     
  5. Mafix

    Mafix Owner Staff Member Administrator Donated!

    you also may have a leak in the booster.
     
  6. 86GTMonkey

    86GTMonkey Well-Known Member

    A/C idle up does not work, stays around 800-900rpm with it turned on. Would a leak in the booster make my pedal feel kinda spongy and have no effect for the first inch or so of travel?

    Thanks for the help
     
  7. dan1_721

    dan1_721 Guest

    Whats your normal idle!? I drop 300 rpm or so with the A/C on....
     
  8. 86GTMonkey

    86GTMonkey Well-Known Member

    Haha, depends on temperature. Warm it's about 700-900 (it drops about 150rpm when I turn on the A/C), cold anywhere from 1k to 2k.
     

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