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  • 11-22-2007, 04:01 AM

    Re: Bad Stumble on Acceleration

    There are two possibilities. One is water somehow got into the spark plug chamber or chambers, there maybe cracks in the rubber boots which has allowed water to enter the chambers causing erratic plug firing. Another cause maybe a leaking windhield gasket, allowing water to seep into the engine ecu located on top of glove compartment, this will cause the injectors to cycle on and off erratically. These two scenarios will cause the exact symptoms that you'll have described.

    88 M6
  • 11-11-2007, 01:35 PM

    Re: Bad Stumble on Acceleration

    I was able to get rid of my stumble off the line. I just did a full tune up including the 02 sensor and altitude sensor.

    I still want to check the AFM under running conditions though.., here's a good reference site for testing. This is for Porsche's but the motronic pin configuration is the same.

  • 11-02-2007, 12:22 AM

    Check both fuel pumps AFM and all engine grounds>>

    Easiest way to check AFM is to swap it out with a known working one. I chased a stumble for a while before discovering a loose ground on the alternator. Tightened it up and runs like new.

  • 10-31-2007, 04:15 PM
    Michael in MD

    Re: Bad Stumble on Acceleration ... Today

    Hi All,

    I have not had time to take the car back into be looked at... but thought I'd give it a spin around the neighborhood today, since it is a beautiful 70 degrees / dry as can be day.

    Yep, you guessed it... runs great... NOT a stumble not once!

    Drove it around and around to try and make it miss... but it didn't.

    Now what? Assume that the wet rainy weather had something to do with it (I'll still have it checked of course) ... thought you'd all like the update (if that is what I can call it)

    Darn... had myself talked into the MAF conversion ;-)

  • 10-29-2007, 10:05 AM
    Michael in MD

    Re: possibilty

    Hi Steve,

    How would I test this, short of swapping in a known good one?

    I have wanted to do the MAF swap for some time now but haven't had the funds (Split Second or Scottie's unit)

    ...this may be a blessing in disguise ;-)

  • 10-28-2007, 06:51 PM


    My M5 had sat for most of a summer a while back, got in it and it absolutely stumbled all over it self everytime I tried to accelerate, probelm was a bad air flow meter
  • 10-28-2007, 04:32 PM
    Michael in MD

    Re: You will get a stumble on acceleration, steady

    Thanks for the reply,

    All new tune up parts including the fuel filter ... less then 2k miles on them.


  • 10-28-2007, 01:13 PM
    Mark 88 M5

    You will get a stumble on acceleration, steady

    state light load may be fine.

    When was the last time the fuel filter was changed?

    Use a stethoscope on the pumps to see if you hear them operating.

    You could install a fuel pressure gauge on the line feeding the injector rail to see if the main pump is working properly.
  • 10-27-2007, 05:10 PM
    Michael in MD

    Re: The O2 sensor electriclal connection is down near

    Thanks Mark...

    I did just have the cat removed and my track pipes put back on. I hope it turns out to be such a simple solution ;-)

    ... but please continue to give suggestions in case its not. I'll let you know what I find.

    btw - What are the symtoms of a fuel pump going bad? What will that feel like as I try to drive it?

    Thanks again,

  • 10-27-2007, 01:18 PM
    Mark 88 M5

    The O2 sensor electriclal connection is down near

    the transmission and it may have gotten wet enough to cause a voltage drop on the circuit. That alone probably wouldn't be a big deal but it is on the same circuit as the fuel pumps. Check and dry that connector and use a bit of dielectric grease in it when you reconnect it.
  • 10-27-2007, 10:26 AM
    Michael in MD

    Bad Stumble on Acceleration

    Hi All,

    Well, caught off guard again... Took the car out yesterday, got about a mile down the road and as I was pulling out of the neighborhood into traffic the car stumbled (choked, wouldn't accelerate)

    I could sort of keep it going by playing with the acc pedal but it stumbled / choked each time after I'd shift gears and get back on the pedal... and seemed okay once in the higher RPM ranges. Still, I turned it around and headed home.

    What are the obvious things to test or change?

    It has been raining here for days, yesterday the hardest of them all but I don't know what this would matter. It ran fine on Thursday ;-)

    Any /all help is appreciated

    Michael (stuck driving the Yukon)

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