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Thread: Idle Hunting when AC is on

  1. #1
    Junior Member
    Join Date:  Jul 2020

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    Idle Hunting when AC is on

    Hello! I’ve done some searching to find anyone with this particular symptom – and I didn’t turn up anything that was quite like this. In case it matters, my O2 sensor and Frequency Valve are not operational.

    Symptoms:
    • My car exhibits the pretty common idle hunt during warm-up. After a minute under load, or a few minutes of holding the throttle slightly open – the car will idle perfectly smoothly.
    • Once idling nicely, if I turn on the Air Conditioning the RPMs are pulled down, and it oscillates just like when it is hunting during warm up.

    I gather the first suspect should be vacuum leak – I plan on a smoke test (I am researching my options for performing this). A full vacuum line replacement is not too expensive. But I was wondering if the AC-related symptoms would lead someone to suspect a particular vacuum line segment / component.

    I can’t find any guidance about how each the vacuum hose segment is replaced. Many are simple and obvious, and I assume the lines that connect directly to the thermotime switch will be a pain (requiring a trip into the VoD), but aside from those – are any vacuum lines particularly challenging? Should I be doing all: engine, and AC / brake related vacuum lines?

  2. #2
    Junior Member
    Join Date:  Jul 2020

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    Edit: Vacuum switch, not thermotime

    How could make such a mistake when these things are so named so clearly

  3. #3
    Senior Member
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    8,290

    My VIN:    10757 1st place Concourse 1998

    You most likely do have vacuum leaks, most Deloreans do. Vacuum leaks reduce the amount of power the idle control system has so the more leaks the less effect the idle system has to control the idle speed. That said, we don't know if the idle system is even working. When the A/C comes on, not only must the motor power the compressor, the alternator must put out more power to run the fans. At idle it is a BIG drag and the idle system must give the engine more power to hold the idle up at 770 RPM. If the idle system isn't up to the task the idle quality suffers. Verify the idle system is working and fix the vacuum leaks. Even with everything perfect, don't expect a perfect idle when the A/C is running, especially if the Lambda system is not functioning.
    David Teitelbaum

  4. #4
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    Thanks David!

    I'm still debating how fancy I wanna go with the smoke test.

    -Does anyone have any recommendations for a vacuum smoke testing product?
    -Where is the best place on the DeLorean to put the smoke in?


    In the mean time I tried some easy free tinkering:

    I removed and cleaned the idle speed motor. Tested it on the bench and it seemed to operate well. I was able to observe it opening completely and then slowly close - with the key ON engine not running.

    I removed, cleaned, and adjusted the idle microswitch and throttle stop. With the throttle fully closed and idle microswitch open, the car idles smoothly at ~1k RPM, and pulls down to ~750 with the headlights and AC on. In the meantime, I may just leave the idle microswitch disconnected - at least then she won't chance stalling due to the hunting when the AC is on at idle.

    I attempted the test where you unplug the outside plug on the idle ECU - the car stalled when I did so. I read it was supposed to tach up to ~2k?

    I'm missing something I think: on a car with no vacuum leaks, when you close the throttle but do not engage the Idle System, the car tachs high - why is this? Where is the air coming from? Is the default state of the idle motor partially open?
    Last edited by deloreandmcxii; 08-07-2022 at 02:00 PM.

  5. #5
    Senior Member
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    8,290

    My VIN:    10757 1st place Concourse 1998

    Quote Originally Posted by deloreandmcxii View Post
    Thanks David!

    I'm still debating how fancy I wanna go with the smoke test.

    -Does anyone have any recommendations for a vacuum smoke testing product?
    -Where is the best place on the DeLorean to put the smoke in?


    In the mean time I tried some easy free tinkering:

    I removed and cleaned the idle speed motor. Tested it on the bench and it seemed to operate well. I was able to observe it opening completely and then slowly close - with the key ON engine not running.

    I removed, cleaned, and adjusted the idle microswitch and throttle stop. With the throttle fully closed and idle microswitch open, the car idles smoothly at ~1k RPM, and pulls down to ~750 with the headlights and AC on. In the meantime, I may just leave the idle microswitch disconnected - at least then she won't chance stalling due to the hunting when the AC is on at idle.

    I attempted the test where you unplug the outside plug on the idle ECU - the car stalled when I did so. I read it was supposed to tach up to ~2k?

    I'm missing something I think: on a car with no vacuum leaks, when you close the throttle but do not engage the Idle System, the car tachs high - why is this? Where is the air coming from? Is the default state of the idle motor partially open?
    There is the problem. It should idle at 750 and stay there even with the A/C and headlights on. You have to figure out why it is idling so high.
    David Teitelbaum

  6. #6
    Junior Member
    Join Date:  Jul 2020

    Posts:    27

    Even with the idle micro switch open? (Disconnected, not completing the circuit even when the throttle is closed)

  7. #7
    Junior Member
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    To restate more clearly:

    With the AC off and the idle microswitch working properly, the car hunts only during warmup and then settles in at ~750

    With the AC off, the car idles at 1k ONLY when the idle microswitch is disconnected (or the striker backed off to not contact the switch).

    With the AC on, after warmup and the idle microswitch working properly, the car hunts between 500-1000, and often stalls at the lower end - particularly when taching down sharply such as when clutching and coming to a stop.

  8. #8
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    8,520

    My VIN:    03572

    With your idle switch disconnected you get ignition advance at idle on a warm engine which is why your idle RPM is higher.

    Since your not running the lambda my guess is your idle motor is sticky causing hunting problems. i recommend the Volvo idle motor over the stock unit. It seems to operate much smoother than the stock unit.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #9
    Junior Member
    Join Date:  Jul 2020

    Posts:    27

    Thank you so much!

    The one I'm seeing online for a b28f has a L shape to the in/out ports - instead of straight thru like DMC. Is this the one you prefer Dave M? Do I need some different rubber hose? Or do you have a link to a source for the one you like?

    Any experience with the DeLoreanGo option? https://www.deloreango.com/us/idle-s...tor-motor.html

  10. #10
    Senior Member
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    8,290

    My VIN:    10757 1st place Concourse 1998

    The idle speed without the idle motor working is not a good indicator of anything. It should be a higher value than when the idle motor is working but there is no spec. I would try to get the Lambda system working now and then the car should behave unless you have a lot of vacuum leaks. Also consider vacuum leaks are cumulative, ie, they all add up meaning a lot of small ones have the same effect as one large one so just because you find one does not mean you don't have others. The most common leaks are the injector seals, the air tube coming out of the bottom of the mixture unit and the seal over the mixture screw. Of course any of the rubber hoses can crack and split making for leaks too.
    David Teitelbaum

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