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Thread: Idle surging/revving

  1. #1
    Senior Member aipri's Avatar
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    Idle surging/revving

    I did some quick searches and didn't find this particular issue, although I may not know exactly what to search for. Anyway last week the car began revving very high during idle. As soon as you start the car it will rev up as if you're giving it gas in park or neutral (it's an auto), the rpm doesn't seem to go up and down its just a steady high rev and loud. Once you put it into drive or reverse the rpms will seem to drop and it quiets down, however I did notice when driving it's accelerating slightly at around 25-30mph even when not giving it gas. I didn't drive it too much like this for obvious reasons. Almost feels like the throttle is stuck but it I looked at it I didn't notice any obvious issue with it. The car also had an aftermarket cruise control put in, sometime in the 90s probably, it hasn't worked in some time though and I wasn't too concerned with getting it fixed. Thoughts? Thanks!
    Anthony

    1983 DMC-12 VIN 16823
    2014 BMW M5

  2. #2
    Delorean Guru
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    Verify that the throttle is opening and closing all the way (with the motor off). Examine the decel springs. Check for any air leaks in the induction system.
    David Teitelbaum

  3. #3
    Senior Member Rich's Avatar
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    It's not an uncommon problem for a car this old. Can't point to an exact thread for it.

    This is potentially a safety issue, especially in an automatic, so make this a priority if you drive it regularly. Your report about it acting like it's on the gas at 25-30mph is a good clue.

    Start by making sure your throttle is actually closed at idle.

    With it idling high in neutral push the arm that's on last part of the throttle linkage forward to be sure it's on its stop and is engaging the black idle microswitch. If the idle drops to normal then we'll work upstream from there to see what's holding the throttle open.
    March '81, 5-speed, black interior

  4. #4
    Senior Member
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    Quote Originally Posted by Rich View Post
    With it idling high in neutral push the arm that's on last part of the throttle linkage forward to be sure it's on its stop and is engaging the black idle microswitch. If the idle drops to normal then we'll work upstream from there to see what's holding the throttle open.
    ^exactly the first thing I would check. I had a failing microswitch that led to your same symptoms. Make sure the screw on the arm is engaging the microswitch (you should hear the click) when the throttle is released (closed position). After that I'd check the connection on the back for continuity with the switch engaged and not engaged.

  5. #5
    Senior Member DMC-81's Avatar
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    Good advice offered so far. I'll add the suggestion to make sure the cruise control system is completely unplugged/disconnected while you are troubleshooting to eliminate that as a cause.
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
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  6. #6
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    What RPM does it sit at while idling high and in park?

    Another possibility (in addition to the good advice you got here already about the throttle spool and cruise control system) is that the electrical connector attaching to the thermistor is either loose or just not making good contact. The thermistor is one of the sensors underneath the intake (accessed by "going into the valley.") It's function is to check for engine coolant temperature and report that back to the idle ECU. If that signal is absent, it defaults to idling high and it is usually a fairly specific RPM, like 2,000 exactly.

    I've had this kind of issue before and it is no fun feeling like the car wants to give itself gas even though your foot isn't on the pedal. Getting into the valley is a pain, but there is a temporary work around you can do that involves making a jumper at the back of the ECU to simulate the signal. Report back on what you found on some of these other suggestions first though before going this route.


    Sept. 81, auto, black interior

  7. #7
    Back to the Omnipresent! Spittybug's Avatar
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    Check your throttle cable. Age/rust/rot can make them frayed and sticky at the point where the cable exits the shield. The shield's end cap can also pop out of the threaded fitting.
    Owen
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  8. #8
    Senior Member AugustneverEnds's Avatar
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    there is a temporary work around you can do that involves making a jumper at the back of the ECU to simulate the signal
    Sorry to hijack but could you point me to where this is explained? thanks
    Nick A.

    1988 BMW 325is
    1982 DeLorean DMC-12

  9. #9
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    Quote Originally Posted by AugustneverEnds View Post
    Sorry to hijack but could you point me to where this is explained? thanks
    No worries.

    Here's a thread that discusses which wires to jumper and some other background on what is going on with that idle ECU.

    http://dmctalk.org/showthread.php?10...s-over-heating

    I've attached a couple pictures of the jumpers I made up. Basically, you need to continue on the proper connection for two of those four spots (as you'll have the white plastic connector pulled away from the ECU) and then those other two spots are what you need to loop a wire back into. You're skipping the thermistor under the intake with this jumper wire. I think it works better in the long run with some sort of resistor inline. Mine kept me going for the balance of the summer and then I went into the valley and fixed it properly that winter.

    ECU thermistor jumper 2.jpgECU thermistor jumper 1.jpg


    Sept. 81, auto, black interior

  10. #10
    Senior Member Bitsyncmaster's Avatar
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    I've tested the ECU and anything under 5 Kohms works the same. So your jumper is fine to bypass and open circuit fault.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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