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Thread: fluctuating idle speed?

  1. #1
    Join Date
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    fluctuating idle speed?

    My idle speed is just below 1k when I start my car, but after a few minutes of warming up, it starts "revving" itself, jumping up to 1200 or so and down to around 500 rpm. When it revs, it does so at a consistent frequency, about 40 to 50 times per minute.

    My first guess was ecu error, but I'd rather not take that out if I don't have to.

    Thanks!

  2. #2
    Join Date
    May 2008
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    Montreal, QC, Canada
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    '85 635 CSi A 5 Sp TCD Stage II
    1st guess should be a vacuum leak
    2nd one is a sticky or misbehaving idle control valve (ICV).
    3rd is bad or errantly connected coolant temp sensor (CTS)

    Bad ECU is last resort

  3. #3
    Join Date
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    Justin Texas
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    2003 525it 2003 X5 4.6is
    Is your TPS adjusted per Bentley?
    I will always own a BMW.

  4. #4
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    82-633CSI 94-325is
    That is a typical symptom when a part of the motronic system is not functioning and thus the idle speed cannot be controlled. All the items mentioned above are things to check. To be even more thorough (if running out of options) look up the Bentley (E28) manual for the inputs at your ecu to confirm operation/connections.

  5. #5
    Join Date
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    JuliustroliJD
    Quote Originally Posted by tlister67 View Post
    That is a typical symptom when a part of the motronic system is not functioning and thus the idle speed cannot be controlled.
    Does this model have the Motronic? I thought it had L Jet.

  6. #6
    Join Date
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    Columbus. IN (Indy 500 area)
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    '87 635Csi & 3 MG's
    It was introduced in '81, depends on the build date.

    If it is Motronic, something is causing the engine to race and when it reaches approx. 1100/1200 rpm the ECU cuts off the fuel to retard it.

    Check for a stuck open ICV, vacuum leaks or mis-adjusted TPS as mentioned.

  7. #7
    Join Date
    Mar 2009
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    82-633CSI 94-325is
    I should have looked closer. Mine is 11/81 build and it is motronic but is E12 otherwise.

  8. #8
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    Yeah, 1/81 here. L-Jet, E12. TPS has been adjusted. New vacuum hoses. I also brought the idle speed down from 1100 to about 800 and the issue seems to have resolved itself.

    Glad to have this sorted out, I was just driving through downtown the other day, and as I was idling at a red light, the car was revving itself and people were looking at me like I was crazy.

  9. #9
    Join Date
    Sep 2008
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    Colorado Springs, CO
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    03 330Xi
    Quote Originally Posted by e24uard View Post
    the car was revving itself and people were looking at me like I was crazy.
    Pretty funny! I am trying to adjust my timing but think that I have to adjust the TPS first? Trying to do that but could not find anything. My TPS is brand new but does not click.
    Rainer
    85 635CSi (Silver, Daily Driver)
    79 633CSi (Euro, Red, New Project)
    03 X5 (Family Vehicle)

    "If you think a good European mechanic is expensive, try a bad one!"

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