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Thread: idle up and down

  1. #1
    Member
    Join Date:  Sep 2019

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    idle up and down

    Hello again, well I have been out driving around and getting to a couple of get togethers with some of the other delorean guys in the area. Great time. Any way the car starts and runs good, drives down the road good. I was working on getting the A/C to work, and yes as of today I believe I have it working. Back to the subject, about the time I was playing with the a/c I developed an up and down idle, cant tell you the amount as the tach is not reading again. Any suggestions on keeping the electrical connection on the dask from coming loose?? Ok, back again, its firing fine, maybe slightly on the rich side, but right from the minute it starts when its 'cold' to even when it is up to temperature. I am thinking possible vacuum leak being as I was playing with the climate control on the dash. I did pull out the radio and inspect the hoses, they all seemed to be connected. Enough bouncing around any suggestions where to start?? Thanks for any feed back.

  2. #2
    Delorean Guru
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    An "erratic" idle, otherwise known as "hunting" is caused by cylinder imbalance. Anything that makes one cylinder not fire as efficiently as any of the others will cause this. Often indicates the need for a tune-up. Among the causes are;
    low compression
    valve adjustment
    dirty fuel injector
    vacuum leak
    spark plugs, ignition wires, cap, rotor
    The causes can also be very "sneaky". On one car the impulse coil-to-reluctor gap was not equal on all of the lobes, causing one cylinder to not fire right. On another car someone used too long a bolt to ground the wires to the intake manifold and it bent the plug at the front of the intake manifold causing a large vacuum leak right next to the front right cylinder making it run very lean. The K-Jet system can't react fast enough so it has a latent delay. Once the Lambda corrects for the imbalance you are already past that cylinder and the system over corrects and then under corrects causing the hunting. Then you have the idle system also trying to maintain a constant RPM and it has it's own latent delay. Bottom line, even on a very well set up PRV you can expect some amount of hunting. That's why the spec is + - 50 RPM. If this happened all of a sudden, my guess is a dirty fuel injector and/or you caused a vacuum leak.
    David Teitelbaum

  3. #3
    Member
    Join Date:  Sep 2019

    Posts:    78

    Quote Originally Posted by David T View Post
    An "erratic" idle, otherwise known as "hunting" is caused by cylinder imbalance. Anything that makes one cylinder not fire as efficiently as any of the others will cause this. Often indicates the need for a tune-up. Among the causes are;
    low compression
    valve adjustment
    dirty fuel injector
    vacuum leak
    spark plugs, ignition wires, cap, rotor
    The causes can also be very "sneaky". On one car the impulse coil-to-reluctor gap was not equal on all of the lobes, causing one cylinder to not fire right. On another car someone used too long a bolt to ground the wires to the intake manifold and it bent the plug at the front of the intake manifold causing a large vacuum leak right next to the front right cylinder making it run very lean. The K-Jet system can't react fast enough so it has a latent delay. Once the Lambda corrects for the imbalance you are already past that cylinder and the system over corrects and then under corrects causing the hunting. Then you have the idle system also trying to maintain a constant RPM and it has it's own latent delay. Bottom line, even on a very well set up PRV you can expect some amount of hunting. That's why the spec is + - 50 RPM. If this happened all of a sudden, my guess is a dirty fuel injector and/or you caused a vacuum leak.
    Thank you, I am strongly suspecting vacuum as it happened almost at the exact time I worked on getting the A/C in running order. Has there been any issue with the climate control unit in the dash itself leaking as such? All the hoses back there do appear connected. And as I mentioned going down the road I dont notice any issue with skipping/missing a beat. Runs smooth, which I would think if it was an injector it would show up most of the time.

  4. #4
    Senior Member
    Join Date:  Jun 2011

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    Cycle through the different HVAC positions using the mode switch and listen for any hissing. Report back what you notice or hear.


    Sept. 81, auto, black interior

  5. #5
    Delorean Guru
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    If the mode switch is tight (difficult to turn) or hisses, it needs to be rebuilt. Try a bottle of Techron to clean the injectors.
    David Teitelbaum

  6. #6
    Administrator Ron's Avatar
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    Location:  North GA

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    Club(s):   (SEDOC) (DCUK)

    Plug the vacuum tap on the back of the left rail/horn to eliminate all vacuum leaks (except the manifold/o-rings).

    Pull the plug wires one at a time to point you to the cylinder(s) with a miss...
    IF it runs well otherwise:
    RPM drop = Good
    RPM raises when making spark jump = Check for fowled plug.
    No change = Check Spark, Air/Fuel, (compression)

  7. #7
    Member
    Join Date:  Sep 2019

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    idle up and down

    Quote Originally Posted by Jonathan View Post
    Cycle through the different HVAC positions using the mode switch and listen for any hissing. Report back what you notice or hear.
    Yes the climate control does in fact turn hard, I dont hear any hissing as such but then I do where hearing aids. I do however hear a difference in how the car runs when I shut the hvac completely off. the hunting is significantly less with it off. Still a slight hunt but not as wild as when the a/c is on. With this I did have the fan relay fail while out and about yesterday, I replaced it and fans work again.

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