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High idle RPM on 550

Discussion in '456/550/575' started by jznd, Mar 18, 2011.

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  1. jznd

    jznd Formula Junior

    Oct 26, 2008
    373
    Hong Kong
    #1 jznd, Mar 18, 2011
    After replacing my alternator, my engine idle RPM has been acting up. On the way home from the shop, idle speed at startup was normal, ~1k RPM. However, once the car got moving, idle RPM (clutch in, braking/coasting to a stop) hovered at 2k RPM. It stayed at 2k RPM while stopped, clutch out, in neutral.

    I tried resetting the ECU by turning off the battery disconnect switch overnight. After the reset, at startup all normal again @ ~1k rpm. But now, idle RPM while moving (e.g. clutch in, braking/coasting to a stop) hovers at 1.5k rpm. Interesting, it does drop back to normal, but only after the car came to a complete stop.

    Any suggestions?

    I wish I was able to help others on this forum, rather than always being the guy asking for help..!
     
  2. 308 GTB

    308 GTB F1 World Champ
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    Barry Wolinsky
    #2 308 GTB, Mar 18, 2011
    I would just drive the car around a bit longer. Maybe the ECU isn't finished re-learning what it forgot.

    Barry
     
  3. tazandjan

    tazandjan Three Time F1 World Champ
    Lifetime Rossa Owner

    Jul 19, 2008
    39,174
    Clarksville, Tennessee
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    Terry H Phillips
    #3 tazandjan, Mar 18, 2011
    Barry could be correct. Erratic idle like that is also symptomatic of vacuum leaks from a hose or the intake manifold working loose. Worth checking, and cheap.

    Taz
    Terry Phillips
     
  4. jznd

    jznd Formula Junior

    Oct 26, 2008
    373
    Hong Kong
    #4 jznd, Mar 22, 2011
    Barry was correct - thanks! The ECU just took a few drives to relearn what it forgot when the alternator crapped out. Now it's back to normal. Coolant temp doesn't climb unusually anymore as well, but could just be the unusually cool weather we're having here.
     
  5. 308 GTB

    308 GTB F1 World Champ
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    Barry Wolinsky
    #5 308 GTB, Mar 24, 2011
    You're welcome, jznd. After my car is serviced and I know the battery was disconnected, my ECU reset routine involves letting the car idle while I reset the time on the clock and the time and preset stations on the radio. Then a few blocks of driving until I get to Classic Coach's "Prova Circuit." A couple of laps around that highway circuit and the ECU has relearned what it had to.

    Barry


    Terry, I think vacuum leaks or loose manifolds would also illuminate the Check Engine Light.

    Barry
     
  6. Axecent

    Axecent Formula 3

    Oct 15, 2008
    1,112
    Central Texas
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    John
    #6 Axecent, Mar 24, 2011
  7. 308 GTB

    308 GTB F1 World Champ
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    Barry Wolinsky
    #7 308 GTB, Mar 24, 2011
    Last edited by a moderator: Sep 7, 2017
    John, the procedure is a bit simpler in the 550. The idling speed adjuster relearning procedure is described in the WSM. Also, each time the engine is started, the ECU regulates itself on the idle position so it's important to start the engine without depressing the accelerator pedal.

    Barry
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