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Thread: Idle surging/revving

  1. #11
    Senior Member aipri's Avatar
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    Thanks for all of the suggestions! I'll start trying to troubleshoot. Unfortunately another issue my car has is that the tach is frozen, so I can't tell what RPM it is at, however I think it's higher than 2000 RPM, it sounds pretty high. FWIW the car has always been garage kept, even before I got it, and only has 10k miles, so there isn't much in the way of corrosion. Looks like I'll be giving myself a crash course on the throttle and linkage.
    Anthony

    1983 DMC-12 VIN 16823
    2014 BMW M5

  2. #12
    Senior Member Rich's Avatar
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    Quote Originally Posted by aipri View Post
    Thanks for all of the suggestions! I'll start trying to troubleshoot. Unfortunately another issue my car has is that the tach is frozen, so I can't tell what RPM it is at, however I think it's higher than 2000 RPM, it sounds pretty high. FWIW the car has always been garage kept, even before I got it, and only has 10k miles, so there isn't much in the way of corrosion. Looks like I'll be giving myself a crash course on the throttle and linkage.
    You don't need the tach for the initial troubleshooting.

    First step is to see if the throttle linkage is fully closed (against the hard stop and the idle microswitch). You can check this with the engine on or off. Push against the top of the upper throttle arm closest to the intake manifold to see if it's bottomed out (closed) or not. If it moves even a mm or two then the throttle isn't normally closed and you need to figure out why.
    March '81, 5-speed, black interior

  3. #13
    Senior Member aipri's Avatar
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    With the holidays over I finally had a chance to take a closer look. The throttle appears to be closing and it looks like screw on the microswitch is making contact properly and doesn't appear to have any play in it when closed. I took the air box off and cleaned the throttle body with some carb cleaner as well as the microswitch. With the air box off and after cleaning it seemed to be revving a little lower but still seems higher than usual. Originally when starting and in park it was idling so high that it sounded like the throttle was like half open. I took a couple pictures as well in case anything looks out of place? I haven't done anything with the cruise control, which wasn't working anyway either. What do you think should be the next step? I also saw some talk about the Idle speed regulator or Idle ECU being an issue that causes the rpms to spike.

    17903120-EB48-45E7-894B-8E7CB96BB6B2.jpgD6DF83BA-95DC-40DD-BE7E-260563B669FD.jpg
    Last edited by aipri; 01-03-2019 at 06:58 PM.
    Anthony

    1983 DMC-12 VIN 16823
    2014 BMW M5

  4. #14
    Senior Member Rich's Avatar
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    Quote Originally Posted by aipri View Post
    The throttle appears to be closing and it looks like screw on the microswitch is making contact properly and doesn't appear to have any play in it when closed. I took the air box off and cleaned the throttle body with some carb cleaner as well as the microswitch. With the air box off and after cleaning it seemed to be revving a little lower but still seems higher than usual.
    Before moving away from the linkage let's review the two things in bold above just because it's where you get the most bang for your buck on this kind of problem.

    For both of these tests start by pushing the accelerator then slowly releasing it to see where the linkage ends up. Helps to have a friend do this while you watch/listen in back with the engine off. Without pushing on the linkage or throttle arm to begin with.

    About the microswitch be sure that not only the screw hits the switch lever but the switch also makes a click at the end of the throttle travel. If you do not hear the switch close then push the linkage by hand to see if it goes the rest of the way and clicks the switch - which indicates a linkage/cable issue.

    Also check - before pushing the throttle arm closed - that the idle arm stop screw is fully against its stop. Use very thin paper like a stiff dollar bill as a feeler. Any gap after means the throttle isn't in idle position. If you do find a gap don't be tempted to fiddle with the idle stop screw. The problem is upstream if there is one.

    With these final checks that the throttle closes by itself every time and the microswitch arm clicks you can move to more complicated tests. This test doesn't rule out a bad switch, only bad actuation of the switch.
    March '81, 5-speed, black interior

  5. #15
    Senior Member aipri's Avatar
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    Quote Originally Posted by Rich View Post
    Before moving away from the linkage let's review the two things in bold above just because it's where you get the most bang for your buck on this kind of problem.

    For both of these tests start by pushing the accelerator then slowly releasing it to see where the linkage ends up. Helps to have a friend do this while you watch/listen in back with the engine off. Without pushing on the linkage or throttle arm to begin with.

    About the microswitch be sure that not only the screw hits the switch lever but the switch also makes a click at the end of the throttle travel. If you do not hear the switch close then push the linkage by hand to see if it goes the rest of the way and clicks the switch - which indicates a linkage/cable issue.

    Also check - before pushing the throttle arm closed - that the idle arm stop screw is fully against its stop. Use very thin paper like a stiff dollar bill as a feeler. Any gap after means the throttle isn't in idle position. If you do find a gap don't be tempted to fiddle with the idle stop screw. The problem is upstream if there is one.

    With these final checks that the throttle closes by itself every time and the microswitch arm clicks you can move to more complicated tests. This test doesn't rule out a bad switch, only bad actuation of the switch.
    Thanks Rich, I did verify that the switch was clicking as well, but it's obviously the original so I'm sure it's possible it could be bad. The revs drop a bit when putting the car into Drive or Reverse, but are much higher in Park and Neutral, not sure if that reveals anything else. I will try the tests you suggested as well.
    Anthony

    1983 DMC-12 VIN 16823
    2014 BMW M5

  6. #16
    Delorean Guru
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    Even if the micro switch clicks it doesn't mean it is good. The only way to really know is to disconnect the wires, hook up a continuity meter to it and see when it clicks that you get continuity. Even that is not a complete test. A complete test would be to get to the plug going into the idle ecu and make sure the switch is connected all the way into the ecu.
    David Teitelbaum

  7. #17
    Senior Member
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    My vehicle was experiencing the same problems. Last night I cut the two thermistor wires from the idle ECU and spliced them together (a technique mentioned in another thread). So far so good. The engine idle is lower and does not surge when placed in park or neutral.

  8. #18
    Senior Member
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    Well the problem is back but I’ve found (hopefully?) the answer: the throttle is sticking and not activating the idle micro switch.

    I had tested this several times with the engine not running and it always check out fine. I could hear the click from the switch every time I let my foot off the gas pedal. Well today it acted up (2,500 RPM in park, no lower than about 1,700 in Drive) and I pulled over, popped the engine cover, and pressed on the idle micro switch. Immediately the idle dropped way down. Took my finger off, idle surged back up.

    So, I’m going to try just lubing the cable, spool, spring, etc before I make any adjustments. Any recommendations for lube?

  9. #19
    Delorean Guru
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    Service Bulletin SC-01-1/82 has a lubrication procedure for the throttle cable. The throttle spool should also be disassembled, cleaned and lubricated. The quadrant link (102555 section 1-4-1) ends wear and when they do you get enough play so that the throttle arm is not positively returned all the way to idle and hit the micro switch. Either replace the link or lengthen it so that it pushes the throttle arm all the way down at idle.
    David Teitelbaum

  10. #20
    Senior Member Bitsyncmaster's Avatar
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    Grease the spring on the cable spool and see if that quick work fixes the problem. It did on mine.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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