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Thread: idle air control test procedure

  1. #21
    Senior Member
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    If the idle speed is increasing and you have ruled out the idle motor, air is getting into the motor somehow. The throttle plates are moving, the decel valves are leaking, somehow more air (and fuel) is getting in. It can also be the advance is kicking in. Tape off the vacuum hose going to the distributor.
    David Teitelbaum

  2. #22
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by David T View Post
    If the idle speed is increasing and you have ruled out the idle motor, air is getting into the motor somehow. The throttle plates are moving, the decel valves are leaking, somehow more air (and fuel) is getting in. It can also be the advance is kicking in. Tape off the vacuum hose going to the distributor.
    Thanks David. The more I think about it, I think there are only a few possibilities remaining. As you said, this is a mechanical system and there are only a few ways this problem can occur. I believe I have ruled out the idle motor. The throttle plates are NOT moving because they are under total tension and unable to move without pressing the gas pedal. My current theory is the springs are getting hot and losing tension thus allowing some air to leak through the decel valves. I am going to seal them closed with a hot glue gun this weekend and see what happens. If the idle keeps coming up but the hot glue seal is unbroken, then it has to be something else.

    Other possibilities:
    1) somehow the ground to the Idle ECU, idle speed motor , or idle speed microswitch is getting interference for a bad connection as the car warms up
    2) The idle motor is somehow crapping out as it gets hot

    I have already pulled off the vacuum hose from the distributor vacuum advance while in the elevated idle condition and there is no vacuum at that time.

    One thing is for sure - I will be the expert at DeLorean idle by the time I figure this one out!
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  3. #23
    Senior Member Rich's Avatar
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    Look for a leak at top of the ISM

    Quote Originally Posted by 82DMC12 View Post

    Other possibilities:
    1) somehow the ground to the Idle ECU, idle speed motor , or idle speed microswitch is getting interference for a bad connection as the car warms up
    2) The idle motor is somehow crapping out as it gets hot
    Picking up on your Other Possibility #2.

    Consider adding this issue to your potential problem list: The top of your ISM has a crack/leak in it? The leak may change with operating time/temp?

    Here is a video posted by jawn101 demonstrating one way it can behave with this failure: https://www.youtube.com/watch?v=JEZbVVOiPfc

    It's not obvious that air leaking into the ISM connector or the top of the ISM body can create a fast or variable idle speed problem. But it can.

    Jawn101's thread which is partially about this is here. You can pick up at about Post #50, also noting another forum user chiming in saying their ISM failed similarly. You'll notice Jon (jawn101) started out chasing the decel springs as a cause of an idle problem...

    I've had my original ISM go bad in a similar way - a vacuum leak near the connector base (I was able to patch the leak, no need to replace the ISM so far).
    March '81, 5-speed, black interior

  4. #24
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Rich View Post
    Picking up on your Other Possibility #2.

    Consider adding this issue to your potential problem list: The top of your ISM has a crack/leak in it? The leak may change with operating time/temp?

    Here is a video posted by jawn101 demonstrating one way it can behave with this failure: https://www.youtube.com/watch?v=JEZbVVOiPfc

    It's not obvious that air leaking into the ISM connector or the top of the ISM body can create a fast or variable idle speed problem. But it can.

    Jawn101's thread which is partially about this is here. You can pick up at about Post #50, also noting another forum user chiming in saying their ISM failed similarly. You'll notice Jon (jawn101) started out chasing the decel springs as a cause of an idle problem...

    I've had my original ISM go bad in a similar way - a vacuum leak near the connector base (I was able to patch the leak, no need to replace the ISM so far).
    Rich,

    Thanks for thinking about me! I did perform a smoke test with both a cold engine and a hot engine and looked specifically for that type of leak. Negative. My next two tests are:

    1) Run a temporary ground for the idle ECU, idle microswitch, RPM relay, and perhaps also the ignition ECU (harder to tap into though) so I can totally eliminate a grounding issue that could be crapping out when the engine gets hot

    2) Got a hot glue gun today so I'm going to temporarily glue to decel valves closed and see if that helps. If it does, I know my problem is the decel springs though they look visually OK. Sounds like there is some ability to tighten the springs down too. Maybe when I was cleaning the butterfly I disturbed the calibration of the springs.

    Hopefully get to this over the weekend - it's been oppressively hot in Kansas City this week (heat index over 100)
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  5. #25
    Senior Member 82DMC12's Avatar
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    To anyone reading this in the future, I did hot glue my decel springs closed and it did NOT fix my creeping up idle when hot. So I think it's safe to say if your springs look OK and they do not appear collapsed with coils touching, they are not the problem.

    I did eventually solve my idle issues but they coincided with a tachometer problem related to white/slate off the impulse coil making a bad signal. Either the impulse coil was bad or the tach fly lead was bad (I replaced both at once). That solved the tach. I also re-set my throttle arm while I had the throttle body out and on reinstallation my idle was fixed too.
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

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