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surging/vibration at idle

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    surging/vibration at idle

    I know this is very difficult to describe through prose but I'm going to give it a try anyway. This is for my E30 (91', 325i, auto-tragic).

    I notice this when I first start the car up, and when I'm at a stop light (so idling situations). The car vibrates and engine sounds like it's throttling up and down causing a subtle surge. The rpms hold steady at 600 rpmson the tach, a little bit higher (700/800) when the engine is cold until a few minutes later it settles to 600. I'm not sure if this is just normal, I drove a few different E30s when I was shopping for one but didn't really notice it, although I'm not always as observant as I'd like to be. The car was like this when I got it. Driving around the engine operates as one would expect and is smooth.

    I've changed/replaced: plugs, O2 sensor, breather hose, air intake boot (all new obviously). I thought the surging was an idle issue which prompted me to clean the ICV and replace the cracked air intake boot. In retrospect my rpms were always constant so I never really had any idling issues to begin with. The car is in good shape for its age and the engine mounts don't look like crap or anything.

    #2
    I've noticed that when my car is warming up, it'll have a lopey idle - almost as if it has a nice cam.

    It just goes away once it's warm.

    7-800 rpms on start-up is just fine and 600 is the normal idle speed for your car.
    Me: "I can't wait to redline my car!"
    Mark: "Didn't you just break a rocker arm?"
    Me: "Yeah, I don't think I've learned my lesson."
    Mark: "You never will."

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      #3
      Yeah, my idle is normal. I think my exhaust has a leak or something, gotta check it out later.

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        #4
        My first suspicion is that you still have intake leaks. Find a shop that can run a smoke test on the intake. There can be other causes of this, but until intake leaks can be ruled out there is no point is per suing the alternatives.
        The car makes it possible, but the driver makes it happen.
        Jim Levie, Huntsville, AL

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          #5
          Or just replace everything vacuum related including your valve cover gasket and you should be fine.

          If it's not a vacuum leak then maybe a sensor like the coolant temp sensor which is cheap and easy to replace.

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