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Would a BAD MAF cause slow idle return?

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    Would a BAD MAF cause slow idle return?

    Well, after some troubleshooting what i thought was a vacuum leak i have had some interesting results.... Ill share them here so that hopefully i can save someone else some time in the future...

    Initial findings:

    engine would go in and out of smooth idle. lots of sucking noise in engine compartment, engine would be very slow to return to idle after reving..

    First step:
    Removed ICV and all accompaning hoses, cleaned out ICV with brake cleaner, lubed, and put everything back together making sure it was all good and tight.

    After First Step:
    Engine had a very large idle hunt that was never experianced before, it would surge between 500 and 1200RPM, throttle response still poor, and returned to hunting after throttle.

    Second Step:
    After reading a bit online, i unplugged the Mass air flow sensor and the o2 sensor, which i have heard on line that both can cause the issues the INITIAL issue i was having with slow throttle return.

    After Second Step:
    Engine settled into a rough idle at 500rpm, was definatly running rich(exhaust had a stronger then normal gas smell)

    Third step:
    Just to make sure i wasnt going crazy, i reinstalled the o2 sensor and MAF

    After Third step:
    Engine fired right up into the crazy hunting pattern it was doing before. so i am not crazy...

    Fourth step:
    Well the engine was running i unplugged each component seperately, the o2 sensor made minimal difference in the engines running. As soon as i pulled the plug on the MAF the engine settled right down to a 500 rpm idle.

    Other interesting observations..
    The crank case vent hose(#5 here http://www.realoem.com/bmw/showparts...84&hg=11&fg=15 ) varried the idle a fair bit, without it hooked up to the connector on the back of the intake the idle would be around 500, when it was hooked up the idle instantly increased to 7-800, no other obervations were made in response times or running characteristics..

    New MAF on the way, will update after i get it...
    Last edited by MattM; 10-02-2009, 10:41 PM.


    Full Build Thread Here

    #2
    You mention a large sucking noise originally, generally that is signs of a vacuum leak, have you tried using intake cleaner or propane to check for vacuum leaks anywhere?

    -Cam
    Cam .W '91 325is

    Spaz's 1991 Alpine White???? S52 Build Thread...

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      #3
      Mattm,
      What happened with this issue?
      Thanks

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        #4
        I'm kind of having similiar issues. Unplugged the MAF and it idled smooth and fine. Swapped out MAFs with my dad's 95 525i MAF, but it did the same thing. Seemed like it idles too low and bounces a lil but once the MAF is unplugged it runs fine and smooth.
        sigpic

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          #5
          it was a vacum leak, make sure you plug the crank case ventalation hose into the throttle body boot, as well as the ICV..

          My MAF was fine.


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            #6
            My car had the slow return to idle issue. It was the TPS.
            I Timothy 2:1-2

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