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Thread: Another high idle thread - how-to on butterfly plate springs?

  1. #1
    My friends think I'm nuts jawn101's Avatar
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    Another high idle thread - how-to on butterfly plate springs?

    Hey all. So, this issue has been plaguing me for years and I've finally decided it's time to tackle it.

    Same old story. Car turns over perfectly every time, initially the idle is a smooth 750.

    After some warm-up or driving, the idle will raise to 900-1000. Blipping the throttle doesn't help it. Shutting the engine off entirely and restarting it, however, does. If I get out and push the throttle arm closed in this state, it doesn't change anything.

    This leads me to believe that it's not mechanical, to do with the quadrant arm, throttle spool, etc at this point, but rather perhaps vacuum related. All of my vacuum hoses are new and silicone (and have been recently done, without helping this issue).

    I remember ages ago Elvis was quite hot on the notion of cleaning and maintaining the decel valves on the butterfly plates. I know that these might stick slightly and cause a higher idle.

    My theory right now is that it makes sense that shutting the engine off entirely and restarting it would remove any air backpressure. This in turn is allowing them to close fully and resetting my timer on the high idle for a bit.

    But now I can't find that thread he created about maintaining those springs. Getting the throttle body out is a fairly quick job, but I can't remember if he was advocating for new springs (and if so where to get them) or a good way to check/revitalize the existing ones.

    Anyone have a link, a guide, or any other advice? My theory may also make no sense and I'm open to that. But this engine has been overhauled several times and all the new plugs, wires, injectors, fuel lines, water/vacuum hoses and gaskets in the world have not changed this one lingering issue.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

  2. #2
    Administrator Ron's Avatar
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    First, double check that the vacuum routing going to the vacuum solenoid is correct. If the two are backwards, it will act normal until you rev the engine, then hold the advance a little high until you shut the engine off because it can't vent correctly.

  3. #3
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Ron View Post
    First, double check that the vacuum routing going to the vacuum solenoid is correct. If the two are backwards, it will act normal until you rev the engine, then hold the advance a little high until you shut the engine off because it can't vent correctly.
    Oooh, interesting. I hate that solenoid on the diagram because it's like...basically impossible to tell

    Will do that now. It should only take an hour
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

  4. #4
    My friends think I'm nuts jawn101's Avatar
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    OK, that was easier than I remembered it being. The confusing part of the diagram I was remembering is actually the CSV tube.

    Solenoid is hooked up correctly.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

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    Senior Member Bitsyncmaster's Avatar
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    Everything but the shutting the engine off fixes it sounds like bad deceleration springs. So how long does that shutting off fix it? The springs should only open after you release the throttle quickly.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    Everything but the shutting the engine off fixes it sounds like bad deceleration springs. So how long does that shutting off fix it? The springs should only open after you release the throttle quickly.
    Uh, not very long. A minute or two maybe?
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

  7. #7
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    Jon, I've surprised myself more than once on how much things improve by cleaning ground connections. If your issue isn't mechanical because pushing the idle microswitch with your finger doesn't change anything, I'm thinking that could leave electrical as the culprit. i.e. the engine is doing this inadvertently because the signals aren't great. The idle control system wires go off in a lot of different directions and get influenced by more than one ground connection. Some that come to mind that could be affecting this on your car are the big jumble of them on top of the intake on the driver's side toward the front of the car. Right beside where the idle speed motor mounts to the intake. That is also where a slim ground wire connects from the ballast resistor bracket. In addition, the O2 sensor ground is the connection on the passenger side of the intake, sometimes hidden by the plastic air filter box. It has a quick connect on the end of a short red wire that screws into the intake (on the side of the aluminum and not the top IIRC). This is where I would start, given what you've described and done so far.


    Sept. 81, auto, black interior

  8. #8
    My friends think I'm nuts jawn101's Avatar
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    Quote Originally Posted by Jonathan View Post
    Jon, I've surprised myself more than once on how much things improve by cleaning ground connections. If your issue isn't mechanical because pushing the idle microswitch with your finger doesn't change anything, I'm thinking that could leave electrical as the culprit. i.e. the engine is doing this inadvertently because the signals aren't great. The idle control system wires go off in a lot of different directions and get influenced by more than one ground connection. Some that come to mind that could be affecting this on your car are the big jumble of them on top of the intake on the driver's side toward the front of the car. Right beside where the idle speed motor mounts to the intake. That is also where a slim ground wire connects from the ballast resistor bracket. In addition, the O2 sensor ground is the connection on the passenger side of the intake, sometimes hidden by the plastic air filter box. It has a quick connect on the end of a short red wire that screws into the intake (on the side of the aluminum and not the top IIRC). This is where I would start, given what you've described and done so far.
    Good thinking, but the idle microswitch is already engaged when the throttle is closed; and if I open the throttle by hand in the engine bay and disengage the switch I can tell that it's definitely having an impact on the running condition. I did the grounds a few years ago in the car and everything else is rock solid, including a direct 2ga cable from the battery directly to the transmission and Dave's relay strip buses.

    I'll still double check the small grounds you mentioned at the top of the intake - they're easily accessible and could easily have been jostled or damaged with all the top end work I've done recently. But from my own observations that doesn't feel right.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

  9. #9
    My friends think I'm nuts jawn101's Avatar
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    Jonathan: quick update, I went and checked those top top end grounds. Everything good to go, I did polish the ring terminal for the 456 bank one but it was really fine, just cosmetic there.

    After another couple start tests (including an unrelated issue with a popped out injector, my fault) it seems that I might be mistaken about shutting off/restarting causing the idle to go back down. I did a few starts in fairly rapid succession and it was sticking in the high 900s each time after the initial warm up at ~775.
    Jon
    1981 DMC-12 #02100. July 1981. 5-speed, black, grooved w/flap.
    restoration log, March 2011 to present
    full and detailed photo restoration log

  10. #10
    Senior Member Bitsyncmaster's Avatar
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    Post a photo of the deceleration springs. You pull the W pipe and move the throttle and you can see them. If they are not evenly spaced, they are bad.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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