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Rough idle only when warm: fuel or air?

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    Rough idle only when warm: fuel or air?

    As the title says, my 91 318i has a relatively smooth idle from a cold start, but if I start it when the engine's already warm, it's rough to the point where it's stalled a couple of times. If I lay on the accelerator, it hesitates for a few seconds, then finally revs, and once I've reved it up to around 3k, it runs pretty smooth. No problems pulling up to redline in all gears once I've gotten it reved up like that.

    Honestly, what this problem feels most like to me is my old motorcycle with carbs instead of fi. Does this sound like an air problem or a fuel problem, or is it impossible to tell without taking things apart? I feel certain that I have some vac leaks, and mess under the intake is on my to do list. But if my injectors or fuel lines need to be messed with, I'd rather know going into it so I can deal with them before putting the intake back together.

    What do you guys think? Ideas? Thanks.

    #2
    That sounds like a vacuum leak to me. When the car starts, it runs richer to compensate for the engine being cold. This will cover up the effects of extra air getting into the system, to an extent. When it warms up, the DME leans out the mixture to the point where there is then too much air and not enough fuel.

    Project M42 Turbo

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      #3
      Thanks for the quick reply - that sounds logical. Time to get after those vacuum leaks!

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        #4
        If its not a vacuum leak look into the AFM. Do you know someone local with a working one you can try?

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