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  1. #1
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    Vacuum leak?

    I've had a slight stumble at throttle and on light throttle recently, and was able to get it to go away at idle, by removing my recently replaced oxygen sensor. The stumble is still there when I held the rpm at around 2500. My thought is that there is an intake leak causing the car to stumble when the o2 sensor leans out the mixture, and it goes away due to it running in open loop when I remove the o2 sensor. I'm double checking here before I go out and replace all the intake rubber.


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    Think plugs.


    '87 <b>528e</b> "is" 214.5k
    '88 <b>M5</b> 193k (Head removed, ready for reassembly)
    Dinan gen.1
    OEM Front & Rear Fuzz-buster

  3. #3
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    Re: Think plugs.

    My maintenance records say the plugs were replaced 10k miles ago, and they look fine. Anything else to check first?

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    when you say it "stumbles", do you mean

    it hesitates to rev up, or that at a constant RPM, it has an occasional miss?
    '87 <b>528e</b> "is" 214.5k
    '88 <b>M5</b> 193k (Head removed, ready for reassembly)
    Dinan gen.1
    OEM Front & Rear Fuzz-buster

  5. #5
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    An occasional miss

    Sorry about the wrong term. There is an occasional (one or more every 2 seconds or so) under constant or light throttle. Maybe it's there under harder throttle and the engine noise just covers it up. If you're an oblivious driver, you would never even notice it was there. There was a stumble with the stock chip, it became an occasional miss after putting in the performance chip (I don't know why).

  6. #6
    Gcracker
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    Start cheap and go from there...

    I'd start with plugs (make sure the gap is exactly right)
    Check your wires for resistance (forget the correct value)
    Check your cap/rotor

    From there, you could watch the O2 sensor output if you have access to a grapher, or you could try unplugging it to see if it stops. Unfortunatley, that may work only because the mixture is enriched, not because you've opened the "loop" and made the ECU ignore sensor inputs in a general way.

    You may check the AFM for smoothness through its range of motion, but I doubt it based on your description. That reminds me, you used the right term, but there are a few ways to stumble. ;)

    Hope this helps get you started!



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    Re: Start cheap and go from there...

    The o2 sensor removal did make it go away at idle, and I was thinking that because it was enriched. I didn't think plugs (or the electrical system in general) cared about the a/f mixture, so I was leaning towards an intake leak that only affects the car when the throttle's down.

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