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Thread: IAC / Aux Thermostat and high Idle

  1. #1
    Join Date
    Jan 2005
    SF Bay Area

    Default IAC / Aux Thermostat and high Idle

    Hi all.
    I've suspected a dirty IAC for a while now because of occasional high/floating idle (1000-1500).
    1992 525i with 270K miles on it.
    Recently the car has been running well. Idle has been down at normal - ~ 700. (Although it failed smog again...thinking cats again).
    I took a trip from SF down to Carmel Valley for some golf this week and on the way down, the idle again became high.
    I've ruled out a vacuum leak because the high idle isn't consistent.
    When cold, idle is normal (for cold start), but when at operating temp, it's high.
    Before I pull it apart to clean out the IAC, I was wondering about the Aux Thermostat.
    My aux thermostat got broken a few years ago so it's bypassed now. I have a new one but haven't installed it yet.
    From everything I've read, the thermo is to keep the throttle plate from freezing in cold weather...something I don't need to worry about here in the SF bay area.
    I got thinking though (dangerous) if the thermo would also act to keep the throttle plate at engine temp so it didn't get too hot in hot weather/driving.
    So I have two questions that I was wondering if you could weigh in on:

    - would a higher than engine temp on the throttle body result in a higher idle?
    - is my assumption that the high idle isn't due to a vacuum leak correct since the high idle isn't consistent?

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    SF Bay Area
    1992 525i/5-spd, 250K mi
    17inch wheels

  2. #2
    Join Date
    Feb 2006
    Oldham, England


    The DME controls the idle control valve. But I'd say it is certainly worth ensuring that the throttle butterfly is closing properly.

    If the rpm do not go down a little when you remove the oil filler cap, you probably have a vacuum leak.
    Oct '00 E46 330i. Feb '92 525i (departed)

  3. #3
    Join Date
    Dec 2004
    Sydney, Australia


    I'd be checking the Idle Control Valve. The aux thermostat is a common bypass, I've never seen it cause idle problems in temperate climates. That said, a coolant temperature throttle body doesn't seem to reduce performance any.

    I'm suspecting you are yet to do a full smoke test to find your leak(?). They can get more difficult to track down with age, but also failing smog is another clue to it being vacuum related.

    Check also that the brake booster is not leaking, in-line purge valve to the carbon canister, they can foul up and stay open but can be cleaned. On M50 the small FPR hose at the back, but most likely are the bastard connector up and around behind the throttle body, and the o-rings leak and like the PCV tube connector going to the front of the valve cover, they shrink/squish down flat with age. If >3 years old, change them. You can buy new connectors cheaply, but new Viton O-rings are all it should need with wipe of silicone lube, unless someone has added RTV/sealer in which case that needs to come off beforehand.

    Not to mention the valve cover gasket (always leak with age).
    Last edited by genphreak; 06-03-2017 at 06:43 PM.

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