'84 4.1 idles up and down when the A/C is on
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HT4100 4.1, 4.5, 4.9 Discussion, '84 4.1 idles up and down when the A/C is on in Cadillac Engine Technical Discussion; And even when the a/c is off, it idles kind of high too - but there's no jitteryness/no miss so ...
  1. #1
    Bobby Eyes is offline Cadillac Owners Member
    Automobile(s): 1984 Coupe deVille
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    '84 4.1 idles up and down when the A/C is on

    And even when the a/c is off, it idles kind of high too - but there's no jitteryness/no miss so I don't think it's the cam
    Any help with diagnosing/fixing the problem would be appreciated, thank you in advance!

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  3. #2
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: '84 4.1 idles up and down when the A/C is on

    Probably load from the compressor clutch engaging is causing the the belt to drag/release, and the ISC reacting to increase/decrease idle speed as appropriate. If you have a high idle in the first place then you probably ought to investigate vacuum leaks, ISC adjustment, timing, etc. You might also be seeing some problems with the A/C compressor if the drag is really high

  4. #3
    Bobby Eyes is offline Cadillac Owners Member
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    Re: '84 4.1 idles up and down when the A/C is on

    Quote Originally Posted by ehall View Post
    Probably load from the compressor clutch engaging is causing the the belt to drag/release, and the ISC reacting to increase/decrease idle speed as appropriate. If you have a high idle in the first place then you probably ought to investigate vacuum leaks, ISC adjustment, timing, etc. You might also be seeing some problems with the A/C compressor if the drag is really high
    So should I look into replacing the compressor?

    Oh and I have another problem that I didn't mention, when stopped at stop lights for example the car wants to take off (it lunges forward) even with my foot on the brake.

  5. #4
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: '84 4.1 idles up and down when the A/C is on

    I would fix the hunting problem before messing with the compressor

    As for the surging, the car shouldn't lurch if the brakes are good. Not sure if you meant something else

  6. #5
    carnut is offline Cadillac Owners Master
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    Re: '84 4.1 idles up and down when the A/C is on

    Without the service manual, determining the base idle and ISC settings would be a guess at best. One simple thing you can do is set the air cleaner assembly over to the side and with the engine running at normal engine temperature, raise the throttle from under the hood with one hand and with the other hand press down FIRMLY on the ISC shaft till it fully retracts then QUICKLY disconnect the wiring connector to it. Then release the throttle slowly. It should then be barely running, idling around 400 RPM. Then with a .060 feeler gauge verify the air gap between the ISC shaft and the throttle lever. If incorrect, rotate the shaft with a pair of pliers to get this .060 air gap. Then shut the car off and reconnect the wiring plug to the ISC. Start the car with the climate system off. Let it run a minute or 2 then set the climate system to 70 degrees and AUTO mode. Watch the ISC shaft as the compressor cycles on the off. Normal operation is the ISC will extend a half second before the compressor engages then retract when the compressor disengages. The point of this is to keep the idle speed very even, within 50 RPM or so as the load of the A/C is added then taken away. If the ISC does not move then its either faulty or has no power to it. Report back once this air gap operation has been done. If this is beyond your expertise, have a shop do this first before any further diagnosis or part replacement.

  7. #6
    carnut is offline Cadillac Owners Master
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    Re: '84 4.1 idles up and down when the A/C is on

    I'm sure you know that before anything is checked and/or adjusted, basic engine timing must be verified. Spec is 10 degrees BTDC with the green lead wire disconnected behind the distributor. If advanced it will idle too fast because the computer will try to advance the timing even at idle.

  8. #7
    cadi509's Avatar
    cadi509 is offline Cadillac Owners Connoisseur
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    Re: '84 4.1 idles up and down when the A/C is on

    My 84 does the same thing with the a/c its probably just the way it runs, my a/c still works perfect

  9. #8
    carnut is offline Cadillac Owners Master
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    Re: '84 4.1 idles up and down when the A/C is on

    I take the time to write on how to diagnose and/or adjust things and someone says its normal?? This car should idle close to 650 in gear A/C on or off if everything is correct. 26 years of operation and back yard mechanics adjusting the ISC without the proper knowledge requires these adjustments be verified.

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