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Thread: High idle speed and no more surging after intake rebuild

  1. #41
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  2. #42
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    If you can find a way to lock yours down to test, that would also work. Maybe some hot glue.
    That's a VERY good idea - hot glue can be removed easily when I'm done testing. First I am going to replace my spark plugs with NOS Bosch HR6DC (same as OEM but copper instead of silver). I wonder if these NGK's which are running too hot are causing crazy things with poor combustion and the idle system is over compensating. Once the new plugs are in, if the idle is still high, I will glue them shut.
    Andy Lien

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

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  3. #43
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    Gluing them shut is OK for testing purposes but if it is a 5 speed you should keep them operable. They serve no function on the automatics. Deloreans are unique in that the specs on the motor ( timing, idle speed, vacuum and mechanical advance) are exactly the same no matter 5-speed or auto. You don't see that on most other cars. The idle speed is supposed to go down as the motor warms up, not increase. My guess is there is looseness (play) in the throttle linkage (especially the rod ends of the quadrant link) and that is allowing the motor to speed up as it gets warmer. Disconnect the thermister and see what happens.
    David Teitelbaum

  4. #44
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by David T View Post
    Gluing them shut is OK for testing purposes but if it is a 5 speed you should keep them operable. They serve no function on the automatics. Deloreans are unique in that the specs on the motor ( timing, idle speed, vacuum and mechanical advance) are exactly the same no matter 5-speed or auto. You don't see that on most other cars. The idle speed is supposed to go down as the motor warms up, not increase. My guess is there is looseness (play) in the throttle linkage (especially the rod ends of the quadrant link) and that is allowing the motor to speed up as it gets warmer. Disconnect the thermister and see what happens.
    I already know my thermistor is showing closed circuit when both cold and hot though the values seem to be a bit lower than I see often reported. I'm at about 8,000 ohm cold and 850 ohm hot.

    What SHOULD happen if I remove one of the pins from the ECU plug that corresponds to the thermistor? My understanding is open circuit on the thermistor results in a high idle, like 3,000 RPM.

    If I wanted to jump the thermistor and close the circuit , would I use a 10K resistor (like about 1 watt) and bridge the two pins on the on the ECU plug? That would simulate a cold engine all the time to the ECU.

    Am I thinking this straight?
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
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  5. #45
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    I already know my thermistor is showing closed circuit when both cold and hot though the values seem to be a bit lower than I see often reported. I'm at about 8,000 ohm cold and 850 ohm hot.

    What SHOULD happen if I remove one of the pins from the ECU plug that corresponds to the thermistor? My understanding is open circuit on the thermistor results in a high idle, like 3,000 RPM.

    If I wanted to jump the thermistor and close the circuit , would I use a 10K resistor (like about 1 watt) and bridge the two pins on the on the ECU plug? That would simulate a cold engine all the time to the ECU.

    Am I thinking this straight?
    Running the thermistor open will only let the idle motor close very little. Quite a bit from fully closed. It's not a controlled RPM so it dependents on the other engine adjustments. Mine ran about 2500 RPM with an open thermistor. The idle RPM is fixed at 775 RPM not dependent on the engine temp. From memory 5 Kohms or below let the idle motor close to it's minimum. The stock ECU never really closed it fully on my car.
    Last edited by Bitsyncmaster; 06-09-2022 at 05:28 PM.
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  6. #46
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Running the thermistor open will only let the idle motor close very little. Quite a bit from fully closed. It's not a controlled RPM so it dependents on the other engine adjustments. Mine ran about 2500 RPM with an open thermistor. The idle RPM is fixed at 775 RPM not dependent on the engine temp. From memory 5 Kohms or below let the idle motor close to it's minimum. The stock ECU never really closed it fully on my car.
    Dave, what would happen if I bridged the two thermistor wires at the white ECU connector? Can I do this with a simple wire tinned at both ends, or should I use a 10K resistor that simulates the thermistor?

    Andy
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  7. #47
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    Dave, what would happen if I bridged the two thermistor wires at the white ECU connector? Can I do this with a simple wire tinned at both ends, or should I use a 10K resistor that simulates the thermistor?

    Andy
    Just do a short circuit. The short circuit will do the same as a 5 Kohm resistor.

    Now it does adjust the start opening when starting the engine with the stock or my ECU so if you find hard starting you may have to return to the thermistor.
    Last edited by Bitsyncmaster; 06-09-2022 at 06:35 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #48
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Just do a short circuit. The short circuit will do the same as a 5 Kohm resistor.

    Now it does adjust the start opening when starting the engine with the stock or my ECU so if you find hard starting you may have to return to the thermistor.
    Thanks Dave. I will try it one more time once I get new spark plugs installed (early next week).
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  9. #49
    Senior Member 82DMC12's Avatar
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    Hey guys,

    I have performed more tests in an attempt to rule out particular components related to the idle speed, particularly the thermistor and the idle speed motor. I believe I have ruled out the thermistor completely, and I have ruled out the idle motor itself however it's still possible there is some kind of interference affecting the signal to it when the engine gets hot.

    Here is what I found last night:

    First I checked the thermistor's 20K ohm value between Black-yellow and Black-Slate on the "inner" plug of the idle ECU (the one with only four wires on it).

    Stone cold engine but hot summer day in my garage - 4.xx on 20K scale (indicating the thermistor is warm enough due to ambient that no "fast idle" is required)

    Test 1 - I pulled out the idle motor but left it plugged into the black plug. I turned the key to ON (Position II) and quickly went back to look at the motor with a flashlight. I saw the "gate" inside had opened completely and after a few seconds it began to retract towards the left (as installed). It closed until there was but a small space the thickness of a slotted screwdriver blade remaining for air to go though.

    Test 2 - I then turned off the key and removed the black-yellow wire from the white plug (insert a small jeweler screwdriver under the female spade and push up while pulling the wire out to release the lock clip). This resulted in open-circuit and lack of continuity to the thermistor. I repeated the test and found the motor will only close about half way.

    Test 3 - I then removed all four wires from the white plug and made a new jumper that connected the terminals that the thermistor goes to on the idle ECU itself, then plugged in the other two wires (black-green and white-slate) being very careful that they were secure and cannot touch. I turned the key to ON and checked the motor - exact same behavior as the first test, closing nearly completely.

    I reinstalled the idle motor and then ran the car up to normal operating temperature with the jumper installed. After about 20 minutes of idling my idle started to climb as I have been trying to solve for weeks now (thermistor totally removed from the circuit so now I know the thermistor is not my issue!). I pulled out the motor once again (quite hot by now!) and repeated Test 3 and found the motor closed to the same nearly closed position even though the idle had been about 1,000 RPM and erratic. So I believe I have effectively ruled out the idle motor itself as defective when both cold and hot.

    I made a video of my car's condition starting from stone cold start to high idle with erratic tachometer needle. The video is about 7 minutes long but should demonstrate perfectly what I'm trying to solve.

    Thoughts????

    https://youtu.be/rJJejVVsPk8
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  10. #50
    Administrator Ron's Avatar
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    FWIW- It seems odd to me that the voltage droped over time, especially since the RPMs rasied too.

    ...not sure how accurate your gauge is, but it should be reading around 14.2V instead of 13V after all that time...
    (May be related to the tach flicker as well??)

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