Extremely Erratic Idle Until Operating Temperature
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HT4100 4.1, 4.5, 4.9 Discussion, Extremely Erratic Idle Until Operating Temperature in Cadillac Engine Technical Discussion; I have a '93 Deville that has a really erratic idle until it gets up to operating temperature. Not just ...
  1. #1
    Enigmaticwraith is offline Cadillac Owners Member
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    Extremely Erratic Idle Until Operating Temperature

    I have a '93 Deville that has a really erratic idle until it gets up to operating temperature. Not just rough, but like the engine is revving itself between maybe 500-2000 RPM. This started occurring about a week after I put in a new ISC motor, spark plug wires, distributor.

    It's possible that the new ISC is bad, but that doesn't make sense to me, as it starts running perfectly fine as soon as it reaches operating temperature. There're no codes except for E30 - ISC Motor Out of Range. I've checked for vacuum leaks, replaced the ECM (due to a separate issue), in the process of which I checked resistance of all the ECM connected solenoids and relays, I checked minimum air and TPS voltage.


    Any ideas would be appreciated; thanks in advance.

    (also the car has about 263,000 miles on it, the engine has around 30,000)

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  3. #2
    carnut is offline Cadillac Owners Master
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    Re: Extremely Erratic Idle Until Operating Temperature

    The ISC adjustment is out of range! An ISC must be adjusted to a minimum air gap when replaced. On a 93 I believe you retract the ISC thru on board diagnostics then with a feeler gauge verify air gap to the throttle shaft.

  4. #3
    Enigmaticwraith is offline Cadillac Owners Member
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    Re: Extremely Erratic Idle Until Operating Temperature

    Quote Originally Posted by carnut View Post
    The ISC adjustment is out of range! An ISC must be adjusted to a minimum air gap when replaced. On a 93 I believe you retract the ISC thru on board diagnostics then with a feeler gauge verify air gap to the throttle shaft.
    Readjusting the ISC is part of the minimum air procedure, which I mentioned I already did. It's set to the specification in the service manual. Thanks for the suggestion, though.

  5. #4
    the recluse is offline Cadillac Owners Connoisseur
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    Re: Extremely Erratic Idle Until Operating Temperature

    MAP? It's prolly the only thing running in open loop...

  6. #5
    Enigmaticwraith is offline Cadillac Owners Member
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    Re: Extremely Erratic Idle Until Operating Temperature

    I found and repaired a vacuum leak at the throttle body that I somehow missed before, but it doesn't seem to have many any difference, other than the engine running a little smoother once it warms up.

    Quote Originally Posted by the recluse View Post
    MAP? It's prolly the only thing running in open loop...
    I did think of that, but why would the engine then run fine once it's warm? Can the computer correct for the MAP sensor readings with the input from the O2 sensor? Still, I haven't checked that yet; I'll do that tomorrow.

  7. #6
    Enigmaticwraith is offline Cadillac Owners Member
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    Re: Extremely Erratic Idle Until Operating Temperature

    So, I finally got around to checking the MAP sensor and it appears to be fine. I also checked to make sure that the new ISC motor wasn't defective, but it checks out fine too. According to the FSM, the coolant temperature sensor and the MAP sensor are the main things running in open loop, but they both seem to be fine. I've pretty much run out of ideas as to what it could be.

    It's worth noting that once and a while it will run fine in open loop, as long as the engine is warm. I also noticed that in the PCM data display, it lists the spark advance as being 58 degrees at idle which seems very wrong to me. However I've checked the base timing multiple times and it's set to 10 degrees BTDC.

  8. #7
    Enigmaticwraith is offline Cadillac Owners Member
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    Re: Extremely Erratic Idle Until Operating Temperature

    Learned something new. The engine still revs up and down even when I disconnect the ISC motor, it's less drastic, but still there.

  9. #8
    the recluse is offline Cadillac Owners Connoisseur
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    Re: Extremely Erratic Idle Until Operating Temperature

    it ought to, unless the ISC is moving in and out and causing the erratic idle other issues are the cause..

  10. #9
    Sredd is offline Cadillac Owners Member
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    Re: Extremely Erratic Idle Until Operating Temperature

    Everyone forgets about the A.I.R. pump system. See if the pipes to the heads are cracked. Found on 83 Rivera ran smoother

  11. #10
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
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    9 month old thread. And no A.I.R. on the 4.9.

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