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  1. #1
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    Stumbling Idle vs. Vacuum vs. Timing

    So I’ve noticed this oddity before, but it’s become more of an issue lately. Statements about timing are as perceived by the fuel gauge hijack. Bare with me here:

    1) When idling, foot on brake, trans in neutral, timing is constantly being adjusted by the DME and the exhaust note is quiet as if wastegates are closed & stumbling idle (slight misfire before DME raises RPMS).

    2) Still idling, foot off brake, trans in neutral, timing still being constantly adjusted by the DME (neutral being small, unpredictable perceived load I imagine), exhaust note is loud as if wastegates are open & idle rpms still stumbling (slight misfire before DME raises RPMs).

    3) Still idling, foot off brake, clutch in (trans not engaged), timing is constant (sits at zero fuel line), exhaust is loud (open wastegate), idle does NOT stumble (NO MISFIRES).

    The same can be replicated by just pressing the brake with the trans disengaged, as if the brake is causing load & fluctuating the WGs with respect to brake pedal position. Brake on would cause quiet exhaust and stumbling idle (timing adjustments). Brake off and timing settles and exhaust note gets louder.

    This was never an issue before, but I recently I can cause it to misfire by using the brake only at idle and in rare cases the RPMs drop to 400, engine chokes and raises up to 800. This is similar to stopping at a red light. As a result, I try to keep my foot off brake at lights to obtain a smooth idle. This shouldn’t be necessary to achieve that.

    Plugs, injectors and coils were changed ~10k ago. Pulled it all off last weekend and appeared fine. Vacuum lines are in good shape. I’m starting to think it may be a weak vacuum pump as I believe the brake servo and WGs ultimately feed from the same pump.

    Do you guys experience the same oddity at idle?
    Last edited by Tzu; 01-03-2012 at 06:51 PM. Reason: paragraphing

  2. #2
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    Damn work computer screws up my paragraphing. I'll have to fix it when I get home.

  3. #3
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    Not sure I understand your explanation...
    Brakes and WG vacuum are linked of course coming from the same vacuum pump source, but I can’t imagine WG position effecting idle. I’ve played with moving WG position from 0 to 60% DC with no noticeable change in idle (except for exhaust note).

    Timing will retard when the clutch is pressed.

    Do your brakes feel normal when driving, or pressing multiple times in a row?

  4. #4
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    I think I understand what you are saying. My car does the same thing, but on it's own. I don't have to press the brake or anything. I think I read something in the past relating this to filling up the vacuum canisters for wastegate control. idk

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    Brakes feel fine. I should probably log my WGs at idle to see whats happening, but thats not the problem.

    As far as I can tell, pressing the brake should have no effect on timing, only vacuum. But clutch in (no trans load), and pressing the brake, timing goes wonky and the engine studders at idle. Take your foot off the brake with clutch still in, and everything settles down for a nice perfect idle.

    It's only been getting worse. Still not critical, but worse than before.

    Vacuum still exists in the canisters even up to 5-10 mins after shutdown. I suppose the next step would be to grab a vacuum gauge from princess auto and make sure vacuum is building. Just wanted to test the waters first to see if you guys are seeing a similar scenario.

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    I'm sure timing is still jumping around with the clutch depressed, but proceed doesn't register below 0deg. Depressing the clutch will retard timing Xdeg for shifting.

    Vacuum source is a separate pump which is only linked to WGs. Try putting WG global setting to 30%... this is 0 DC and not using any vacuum.

  7. #7
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    nothing to do with the procede, this is how the car works. Flash/piggy/stock, with the clutch in you get smoother timing at idle

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    I'm certain it is jumping around. But if its not in the positives then physically the engine will be more stable, but more wasteful in fuel consumption.

    Laloosh, yes it does happen both tuned and untuned. Still unsure why siphoning some vacuum for the brake servo would cause positive timing advance.

  9. #9
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    Click here to enlarge Originally Posted by Tzu Click here to enlarge
    I'm certain it is jumping around. But if its not in the positives then physically the engine will be more stable, but more wasteful in fuel consumption.

    Laloosh, yes it does happen both tuned and untuned. Still unsure why siphoning some vacuum for the brake servo would cause positive timing advance.
    It could be as simply as thats how the ecu reacts to the clutch switch.

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