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Thread: High idle (2000 RPM) once engine is warmed up

  1. #1
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    High idle (2000 RPM) once engine is warmed up

    Ok, here is the scenario and I've tracked the issue down a poor connection to the thermistor in the Y pipe.

    When I start the engine, it runs normal, around 750 rpm and it purrs.

    When i let the engine idle and reach around 150-160 degrees (going off the temp gauge in the cluster), it's like a switch kicks in and the engine just goes into high idle without me touching anything. If I let the engine cool down, back to normal until the temp reaches 150-160 again.

    - Measuring from the Black/Yellow wire and Black/Slate wire at the IDLE ECU plug (left plug with only 4 wires going to it), resistance to the thermistor in the Y pipe is normal cold BUT when hot, it goes open (no resistance). If I squeeze my hand down towards the back of the engine, under the air intake and "wiggle" the wire harness, I can get it to briefly register 14k ohms (normal should be around 12k-14k when hot) when the engine is hot. Let the wire go and it goes open again. It's weird, when the engine is cold, I can get an 8k measurement and the engine idles fine cold. Before I dig into the VOD, could the thermistor be bad where it has an intermittent connection once it gets hot? I am leaning towards a fault in the connector itself (weird that it's fine cold) but wanted to make sure that the thermistor could cause the same thing.

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    There are several systems that operate based on the engine temperature. Among them is the vacuum advance, and as you mention, the idle system. If the hoses are hooked up wrong to the vacuum solenoid you can get this problem. I would not suspect the idle thermistor because it is supposed to slow the motor down when it gets hot, not speed it up. Besides, it doesn't speed the motor up all that much.
    David Teitelbaum

  3. #3
    Member Azar's Avatar
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    I think I red recently (FB?) that the idle switch was not engaging properly and the idle was staying very hight.

    Gesendet von meinem Mi 9T mit Tapatalk

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    Senior Member Bitsyncmaster's Avatar
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    What the thermistor does to the stock idle ECU is limit how much the idle motor will close. High resistance (cold about 50 Kohms) and the idle motor will probably not close much past 1/4 way open. When the engine is hot the resistance is about 1 Kohms and then the idle motor can close to about 1/16 way open. So it's not a controlled RPM change and will result in different idle RPM on each car depending on curb idle and other settings.

    It's really not needed for the stock ECU to get higher cold idle RPM unless your running is sub zero weather.

    It sounds like you have a bad connection or a bad thermistor since your not getting anywhere close to the 1 Kohm on the hot engine. You can just short the wires at the ECU until you find the real problem.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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    Quote Originally Posted by Bitsyncmaster View Post
    What the thermistor does to the stock idle ECU is limit how much the idle motor will close. High resistance (cold about 50 Kohms) and the idle motor will probably not close much past 1/4 way open. When the engine is hot the resistance is about 1 Kohms and then the idle motor can close to about 1/16 way open. So it's not a controlled RPM change and will result in different idle RPM on each car depending on curb idle and other settings.

    It's really not needed for the stock ECU to get higher cold idle RPM unless your running is sub zero weather.

    It sounds like you have a bad connection or a bad thermistor since your not getting anywhere close to the 1 Kohm on the hot engine. You can just short the wires at the ECU until you find the real problem.
    When I re-routed the main engine wire harness, I accidentally yanked the connector off of the thermistor and didn't realize it. Tried starting the car up after finishing the wires up and from a dead cold engine, the RPM shot up to 2000 RPM. Once I realized what had happened, I got into the VOD, and sure enough, it was unplugged and I plugged it back in. Started the engine up after plugging it in and normal idle. This issue with it spiking to 2000 after the engine heating up was similar in issue which is why I measured the resistance from the Idle ECU plug. The connection is probably "just there" (pin is pushed into the housing or broke) and when the engine heats up and the plastic connector housing begins to warm up, the connection is broken which would cause the ISM to swing wide open which would increase the RPM as if it was a major vacuum leak.

    The wire didn't have a lot of slack in it and there was a little strain on the connector when I was last in there. The pigtail EV1 connector I purchased should alleviate that issue with some extra wire.

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    Quote Originally Posted by David T View Post
    There are several systems that operate based on the engine temperature. Among them is the vacuum advance, and as you mention, the idle system. If the hoses are hooked up wrong to the vacuum solenoid you can get this problem. I would not suspect the idle thermistor because it is supposed to slow the motor down when it gets hot, not speed it up. Besides, it doesn't speed the motor up all that much.
    Vacuum switch is connected properly. If I unplug the advance solenoid when the RPM's are around 2000, the RPM will actually go higher, about 2500. Plug it back in and it drops back down to the 2000rpm. Also, the Vacuum at the WUR is non existent once the engine is warmed up which indicates the thermo vacuum switch is functioning and connected properly.

    I am 95% certain that the issue is the connection to the thermistor from all the tests I ran before the issues starts and after it shoots up to 2000 rpm. If the thermistor circuit goes open, the ISM would swing open allowing more air in and act like a big vacuum leak.

    I just wanted to see if the thermistor itself could exhibit this same issue (going open) when warmed up if it was going bad.
    Last edited by dmcman73; 07-11-2021 at 06:53 AM.

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    Quote Originally Posted by Azar View Post
    I think I red recently (FB?) that the idle switch was not engaging properly and the idle was staying very hight.

    Gesendet von meinem Mi 9T mit Tapatalk
    Mine engages properly and the switch works. I connected the positive lead of my meter onto an "always on" 12v source and then connected the negative lead of the meter to the Black/Green wire on the Idle ECU plug. Throttle plate closed, I get 12v, open the throttle plate a little until the switch disengages, I read no voltage. Close the plate again and I get a 12v reading again. That verifies that the harness, connectors, the switch and the stop screw are all good.

  8. #8
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    Well then bridge the two wires of the thermistor at the ECU connector.


    Rerouted the harness ? why ?
    to make the engine look cleaner and let the harness fail earlier ?
    PVC doesn't last forever - the hotter - the earlier it deteriorates...

  9. #9
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    Quote Originally Posted by Elvis View Post
    Well then bridge the two wires of the thermistor at the ECU connector.


    Rerouted the harness ? why ?
    to make the engine look cleaner and let the harness fail earlier ?
    PVC doesn't last forever - the hotter - the earlier it deteriorates...
    - Bridging the two wires at the ECU not the answer or the fix, the ECU is looking for a resistance value not a short or an open. I have a new connector coming in.

    - I rerouted the wires:

    A. To clean up the engine bay
    B. remove the long brown wires as the later harness did
    C. And a number of other reasons such as making it easier to work on the engine later, improve reliability, etc...

    I'm not sure why you're stating it will fail faster by rerouting the harness like the later car harnesses were, are you saying that the later car harnesses are going to all fail faster VS how the early car harnesses were routed? All my wiring has been rebuilt and rerouted away from direct heat sources with all new connectors (some with improved weather pack connectors where applicable) and aluminum/fiberglass heat shield sleeves for wires that go near high heat areas. The only connector that wasn't replaced was the one to the thermistor which will be replaced this week. All power wires have been replaced with 4 gauge silicone jacket wire as well as heat wrap for the ones going to the starter.

    But thank you for your enlightened and expert input.

  10. #10
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    Quote Originally Posted by dmcman73 View Post
    Vacuum switch is connected properly. If I unplug the advance solenoid when the RPM's are around 2000, the RPM will actually go higher, about 2500. Plug it back in and it drops back down to the 2000rpm. Also, the Vacuum at the WUR is non existent once the engine is warmed up which indicates the thermo vacuum switch is functioning and connected properly.
    FWIW, and just in case you and DT are not on the same page RE vacuum switch v/s Vacuum solenoid...
    If you hook the solenoid's vacuum lines up backwards, some will not vent when the power is cut to it. So once you hit the throttle, the advance will be held high, until you shut the engine off (as when you let it cool down...). This may make it act a lot like what you are saying (but not all of it ;-)

    Quote Originally Posted by dmcman73 View Post
    I am 95% certain that the issue is the connection to the thermistor from all the tests I ran before the issues starts and after it shoots up to 2000 rpm. If the thermistor circuit goes open, the ISM would swing open allowing more air in and act like a big vacuum leak.
    I just wanted to see if the thermistor itself could exhibit this same issue (going open) when warmed up if it was going bad.
    Sounds right.
    I remember reading where sixty something percent of thermistor failures are due to an open condition...But I don't remember it saying anything about those failures being intermittent /with temp.
    Sounds like a bad connection whether it is the plug-socket or one of the leads going inside the thermistor itself loosing contact when hot, or both.
    BUT, yeah, it is weird, since the resistance and temp rise-fall are normally inverted but you're getting 8K cold and 14K hot, when you can get it to read.

    (If it wasn't for the labor risk for a $35 part, I'd offer send you a pull-off for the shipping ;-)

    EDIT: I like the idea of bridging, just to see if it acts consistent...

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