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Thread: Idle Backwards

  1. #1
    Senior Member
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    Idle Backwards

    So here is a problem I have had for some time now. When the engine is COLD the engine idles at 775 RPMs. Then when it gets HOT it idles at 1,500 RPMs. So where do I start testing to see what is bad now?

  2. #2
    Senior Member r00b's Avatar
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    Check if you're idle speed motor is stuck.

  3. #3
    Senior Member Bitsyncmaster's Avatar
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    Look at your deceleration valve springs. If the springs are not evenly spaced, you have the bad ones most likely.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
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    Quote Originally Posted by Bitsyncmaster View Post
    Look at your deceleration valve springs. If the springs are not evenly spaced, you have the bad ones most likely.
    Well first off, I have no idea where they are located to even check them, dummy me. Second, in trying to find them on the internet, etc., I read that they do nothing fir an automatic transmission. Mine is an automatic. Maybe should have said that in the beginning. Is this correct or should I still be checking?

  5. #5
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    Quote Originally Posted by r00b View Post
    Check if you're idle speed motor is stuck.
    The car starts at the low 775 RPM and runs kind of rough. Then when it hits a higher temperature, it runs at 1,000 - 1,200 RPM and real smooth. Both times when cold and hot I disconnected the electrical plug from the idle speed motor - no change whatsoever in the RPM. I'm guessing that means it has issues? Checking here because it is such a pain to get out. The microswitch is set to connect when at no foot pedle.

  6. #6
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Ricker View Post
    The car starts at the low 775 RPM and runs kind of rough. Then when it hits a higher temperature, it runs at 1,000 - 1,200 RPM and real smooth. Both times when cold and hot I disconnected the electrical plug from the idle speed motor - no change whatsoever in the RPM. I'm guessing that means it has issues? Checking here because it is such a pain to get out. The microswitch is set to connect when at no foot pedle.
    I had the same problem. It held 775 RPM until it warmed up and then ran at 1000 RPM even with the curb idle screw backed all the way out. The problem was the deceleration valve springs. I got a replacement throttle body and that cured the problem.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #7
    Motors about after dark Michael's Avatar
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    I would first start with basics (especially if this happened recently), before looking at springs in the throttle body which rarely go bad. check to see if the idle circuit is working, or at least energizing. Secondly, check your vacuum lines, especially at ISM, valve cover back where the distributor advance solenoid is and make sure the plug is on.

    As far as the idle circuit goes, make sure the microswitch is engaging and check for continuity. Deceleration springs and ISM's can go bad, but it's not common and probably not the cause here. I would suspect a bad idle computer before any of that.
    Last edited by Michael; 01-21-2024 at 03:23 PM.
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  8. #8
    Senior Member 82DMC12's Avatar
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    I had a similar problem once, after doing a valley rebuild. Turned out the throttle linkage was just not set up quite right. It would do this reliably if I started the engine cold and touched NOTHING... after a few minutes the idle would very slowly start to come up and stick around 1,000 or higher can't remember exactly where but I have a thread on here. I had to disconnect all the linkage and start from square one with all the adjustments. That fixed it.
    Andy Lien

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  9. #9
    Senior Member glockworks21's Avatar
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    ^^^

    same. I think Andy actually helped me when I had this problem. I changed throttle spool springs and cleaned up cable. It was sticking. then adjusted the cable on the spool to get the right tension.

  10. #10
    Senior Member r00b's Avatar
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    Quote Originally Posted by Ricker View Post
    The car starts at the low 775 RPM and runs kind of rough. Then when it hits a higher temperature, it runs at 1,000 - 1,200 RPM and real smooth. Both times when cold and hot I disconnected the electrical plug from the idle speed motor - no change whatsoever in the RPM. I'm guessing that means it has issues? Checking here because it is such a pain to get out. The microswitch is set to connect when at no foot pedle.
    My engine would start out with a normal RPM and once warmed up it would idle high. It didn't idle rough when first started though. The problem was my idle speed motor. Before it started, could moisture have gotten into the engine? You can take it off and look inside, if the rotating door in the ISM moves smoothly then the problem is probably not the ISM. If it is the problem you can get a replacement on amazon for around $40.

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