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

  1. #1
    Senior Member 82DMC12's Avatar
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    High idle speed and no more surging after intake rebuild

    Over the last couple of weeks I've been cleaning up and restoring my intake system including the throttle body, W pipe, and air meter unit. Basically went down to the valley and rebuilt from the bottom up. I have everything re-installed and now I have a high idle.

    Symptoms include:

    • No surging when cold anymore
    • Idle is starting around 950 and creeping to 1200 by the time the fans come on
    • If I unplug the idle speed motor plug, the engine doesn't die, in fact nothing happens. I thought the motor should die at idle if the plug is removed? This seems to suggest to me that the throttle isn't closed or there is a vacuum leak
    • If I manually rev the engine by standing behind the car and turning either the throttle spool, or disconnecting the linkage and pulling back on the arm connected to the throttle, it doesn't always return to the idle speed it was previously at. Sometimes I have to press the arm into the throttle stop to get it to go that last millimeter. It seems like the butterfly plates are sort of hanging up? I have tried disconnecting the linkage to confirm it's not binding and it doesn't improve the situation.


    One question I have is, should the butterfly plates close 100% so that they are perpendicular in the throttle? Mine are open a few degrees and if you try to turn them a bit further to perpendicular, it feels like it's binding somewhere (this is even if the arm and spring are removed from the throttle and I'm just turning the threaded end).

    I have attempted three times to set the idle by backing out the stop screw and idle switch screw and just barely screwing it in until it hits the stop bracket but I'm not returning to that low 750 RPM like I used to. I'm just stuck at 1000 or greater, and sometimes when revving the engine it gets stuck even higher until I push on the throttle arm to get it to close that last millimeter because the spring on the throttle isn't quite strong enough.

    I did a lot of cleaning with carb cleaner, etc in the throttle. The decel springs appear to be in good shape and I did confirm they are fully seated closed the last time I pulled the W-pipe off tonight. New o-rings on the W pipe and new white plastic spacers in front of them. New paper gaskets on the W-pipe and everything is confirmed tight. I have gone over all vacuum lines and vacuum leak areas and can't find anything however I haven't tried spraying starting fluid or anything around yet to see if it would get sucked in.

    I guess first thing is to figure out why the Idle speed motor won't kill the engine when I unplug it, and if the butterfly plates are supposed to be truly perpendicular?
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  2. #2
    Mad scientist DrWin's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    One question I have is, should the butterfly plates close 100% so that they are perpendicular in the throttle? Mine are open a few degrees and if you try to turn them a bit further to perpendicular, it feels like it's binding somewhere (this is even if the arm and spring are removed from the throttle and I'm just turning the threaded end).
    For what it is worth, this is what my butterfly plates looks like closed: (I am doing a VOD dive too at this time.)
    I don't know if this is how it's supposed to be, when mounted on the engine, but I think mine looks like they close all the way. (When not in the engine.)
    DSC_0499.jpg
    Please excuse the crudity of this DeLorean as I didn't have time to repair it yet.
    VIN 10207 - December '81, Gray Interior, 3-speed automatic, stock PRV engine.

  3. #3
    Senior Member Bitsyncmaster's Avatar
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    The throttle plates should not have to jamb shut all the way. If they do it tends to make them stick closed. I found greasing the throttle return spring on the throttle body is very helpful getting the throttle to return to normal closed. I bet this is your problem since you said you cleaned those parts. You can also increase the spool spring pressure if it is not already in the last notch.

    You probably should check your dwell again since any work on the intake can change the setting.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by DrWin View Post
    For what it is worth, this is what my butterfly plates looks like closed: (I am doing a VOD dive too at this time.)
    I don't know if this is how it's supposed to be, when mounted on the engine, but I think mine looks like they close all the way. (When not in the engine.)
    DSC_0499.jpg
    Thanks for that. Judging by your photo. I'd say mine are at the same not-quite-perpendicular angle in the rest position.

    PXL_20220119_130539015.jpg
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  5. #5
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    The throttle plates should not have to jamb shut all the way. If they do it tends to make them stick closed. I found greasing the throttle return spring on the throttle body is very helpful getting the throttle to return to normal closed. I bet this is your problem since you said you cleaned those parts. You can also increase the spool spring pressure if it is not already in the last notch.

    You probably should check your dwell again since any work on the intake can change the setting.
    Thanks Dave. Yes I had really cleaned all the springs with shop towels to get the grime out but didn't add extra grease again. This morning I removed the throttle from the car, removed the arm and spring, and sprayed some silicone lube into the shaft area, working the spindle back and forth and it feels very smooth. Then I put a thick coating of grease on the nub the spring fits over as well as inside and outside the spring. Did the same for the two springs on the throttle spool. The throttle spool is on the second spring stop already. I can't grease it any more! LOL

    Last night I did set the dwell even at the higher idle so that I'm not dying from gas fumes.... so the CO is actually quite close right now.

    I didn't have time this morning to check the idle after the grease job.... had to go to work. Might be able to try again after work today.

    I'm still concerned about the idle motor. I am positive that the idle motor used to kill my engine if I was at idle and I unplugged it. I tried it three or four times yesterday while the engine was idling high and it made no difference, like zero change. I turned the car off and put the ignition in Run (second position) and the idle motor is humming/vibrating. Maybe I will try some of the test procedures found here http://dmctalk.org/showthread.php?11...test-procedure

    I think if the idle air valve is hanging up or stuck too car open, that would have the same effect as the throttle being cracked open. Is that right?
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  6. #6
    Senior Member
    Join Date:  Aug 2018

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    Quote Originally Posted by 82DMC12 View Post
    Thanks Dave. Yes I had really cleaned all the springs with shop towels to get the grime out but didn't add extra grease again. This morning I removed the throttle from the car, removed the arm and spring, and sprayed some silicone lube into the shaft area, working the spindle back and forth and it feels very smooth. Then I put a thick coating of grease on the nub the spring fits over as well as inside and outside the spring. Did the same for the two springs on the throttle spool. The throttle spool is on the second spring stop already. I can't grease it any more! LOL

    Last night I did set the dwell even at the higher idle so that I'm not dying from gas fumes.... so the CO is actually quite close right now.

    I didn't have time this morning to check the idle after the grease job.... had to go to work. Might be able to try again after work today.

    I'm still concerned about the idle motor. I am positive that the idle motor used to kill my engine if I was at idle and I unplugged it. I tried it three or four times yesterday while the engine was idling high and it made no difference, like zero change. I turned the car off and put the ignition in Run (second position) and the idle motor is humming/vibrating. Maybe I will try some of the test procedures found here http://dmctalk.org/showthread.php?11...test-procedure

    I think if the idle air valve is hanging up or stuck too car open, that would have the same effect as the throttle being cracked open. Is that right?
    Iíve unplugged my IAC valve several times and it never stopped the engine. IIRC it helps keep the engine going under different loads (AC on for example) but will not starve the engine of air if it closes completely.

  7. #7
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by CFI View Post
    Iíve unplugged my IAC valve several times and it never stopped the engine. IIRC it helps keep the engine going under different loads (AC on for example) but will not starve the engine of air if it closes completely.
    Well now you have me confused, Perhaps that's not an accurate test. If you pull the IAC motor connector, does the motor close or does it stay in the position it was previously in? I thought it closed, choked off the air, and killed the engine. I could be wrong. I haven't had to deal with idle issues in a very long time so I'm having to relearn some of this stuff.
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  8. #8
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by 82DMC12 View Post
    Well now you have me confused, Perhaps that's not an accurate test. If you pull the IAC motor connector, does the motor close or does it stay in the position it was previously in? I thought it closed, choked off the air, and killed the engine. I could be wrong. I haven't had to deal with idle issues in a very long time so I'm having to relearn some of this stuff.
    There is no spring action in the idle motor. So when you pull power it should stay put but vacuum forces and engine vibration may change the position. If you happen to pull the connector slowly or at an angle then that may power one way open or closed.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #9
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    There is no spring action in the idle motor. So when you pull power it should stay put but vacuum forces and engine vibration may change the position. If you happen to pull the connector slowly or at an angle then that may power one way open or closed.
    OK so let me ask this. I want to make sure I'm not chasing two problems at once. The first thing I noticed on starting the car, in my cold garage, is the idle did not do the "hunt" it normally does when stone cold with low ambient temp. I'm originally from North Dakota and now living in Kansas where it can easily be 10F to 40F during the day at this time year. For 20 years, my car has had the regular surging hunt when the engine is COLD, like below 40F for sure. I would turn the key, it would crank a couple times, start, and then rhythmically rev to maybe 1100, fall back down to around 750, rev again, fall back down, etc in a very regular pattern until apparently something warms up and the amount of rev diminishes for next two or three revs (like maybe only to 1000, then only to 900, then to 800, then settle in) until the rev meets the idle speed of 750 RPM. Then I have always had a very stable idle. This hunt doesn't happen on a hot or warm engine and doesn't happen at all on a cold engine on a hot summer day. Only cold engine with cold ambient.

    I find posts on here from fairly knowledgeable people saying the engine should never hunt, and if it does, that means you have a vacuum leak. Then I find other smart people saying yes actually it does, and if it doesn't, your lambda or IAC isn't working right. I thought this cold hunt was normal and characteristic of DeLoreans with an intact lambda and IAC system.

    If it's NOT normal, I need to know that so that I don't go looking for why it's not hunting if it's not supposed to anyway.
    Andy Lien

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

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  10. #10
    Senior Member
    Join Date:  Aug 2018

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    Quote Originally Posted by 82DMC12 View Post
    OK so let me ask this. I want to make sure I'm not chasing two problems at once. The first thing I noticed on starting the car, in my cold garage, is the idle did not do the "hunt" it normally does when stone cold with low ambient temp. I'm originally from North Dakota and now living in Kansas where it can easily be 10F to 40F during the day at this time year. For 20 years, my car has had the regular surging hunt when the engine is COLD, like below 40F for sure. I would turn the key, it would crank a couple times, start, and then rhythmically rev to maybe 1100, fall back down to around 750, rev again, fall back down, etc in a very regular pattern until apparently something warms up and the amount of rev diminishes for next two or three revs (like maybe only to 1000, then only to 900, then to 800, then settle in) until the rev meets the idle speed of 750 RPM. Then I have always had a very stable idle. This hunt doesn't happen on a hot or warm engine and doesn't happen at all on a cold engine on a hot summer day. Only cold engine with cold ambient.

    I find posts on here from fairly knowledgeable people saying the engine should never hunt, and if it does, that means you have a vacuum leak. Then I find other smart people saying yes actually it does, and if it doesn't, your lambda or IAC isn't working right. I thought this cold hunt was normal and characteristic of DeLoreans with an intact lambda and IAC system.

    If it's NOT normal, I need to know that so that I don't go looking for why it's not hunting if it's not supposed to anyway.
    If everything is working correctly it shouldnít hunt. Not much anyway. The problem is idle hunting is so common and can happen for so many reasons that most people just live with it.

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