Bucking at 5k RPM after 15 or so mins of driving.

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  • onthames
    Noobie
    • Aug 2021
    • 6

    #1

    Bucking at 5k RPM after 15 or so mins of driving.

    Ok, please help me because I am struggling here.

    Car is a 88' 325i SE UK spec

    When I first drive the car, for the first 15-20 mins it's fine, can hit redline no problems, lots of power and drives fantastic.
    Idles perfectly and smooth always, cold or hot.
    After 20 or so mins I can't get above 5k RPM.
    10 mins after that I can't get above 4k RPM
    10 mins after that I can't get above 3k RPM
    Quickly restarting the car allows me to reach redline again, but only for around 5 minutes then the problem returns.
    NOTE: In neutral I can hit redline all of them time no issues


    TPS tested and working fine.
    Fuel fitler, pump and tank replaced.
    AFM smooth and not stuck anywhere
    No vac leaks (as far as I can tell with my crude setup)

    What I don't understand is why this only happens after an amount of time.
    At first, I thought it was gunk getting stuck on the fuel pump sock filter that built up over the course of a drive and caused RPM limit to decrease over time (as more gunk got stuck on) but this isn't the case.

    I guess it's temperature related? Temp gauge never moves above the 1st bar for reference.

    Video of the problem happening: https://streamable.com/e/n20q9g

    Somebody help me please... This is my only car to commute with and it makes joining the motorway very difficult (lack of acceleration) - it's actually somewhat dangerous because I don't know when its gonna start happening and sometimes I need to accelerate hard match my speed when joining (we have some very very short slip roads here).
    Last edited by onthames; 08-03-2021, 05:52 AM.
  • Gregs///M
    Forum Sponsor
    • Mar 2012
    • 2459

    #2
    Definitely sounds like a failing AFM. Just because the AFM door operates smoothly does not mean the internal electronics are not corroded with 30+ years of oxidation and the spring calibration has not drifted out of calibration.
    Owner - Bavarian Restoration
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    • onthames
      Noobie
      • Aug 2021
      • 6

      #3
      Originally posted by Gregs///M
      Definitely sounds like a failing AFM. Just because the AFM door operates smoothly does not mean the internal electronics are not corroded with 30+ years of oxidation and the spring calibration has not drifted out of calibration.

      Was smooth, jolting is cos my fingers aren't long enough to reach in there properly

      Comment

      • ADEN
        Mod Crazy
        • Feb 2008
        • 620

        #4
        Try to be easy on the throttle not wot and see if it reach 5k then your TPS wot switch is not making contact.

        Google how to test tps wot switch for continuity do it when engine is hot.

        Comment

        • onthames
          Noobie
          • Aug 2021
          • 6

          #5
          Have tested TPS but engine wasn't particularly hot. I'll test again when hot

          Comment

          • ADEN
            Mod Crazy
            • Feb 2008
            • 620

            #6
            Oil inside tps react to temp lol.

            I was unable to reach beyond 2k when cold until tps gets hot (10 mins) then i can rev freely.

            Comment

            • onthames
              Noobie
              • Aug 2021
              • 6

              #7
              Fixed. I took the cover off the afm and cleaned all contacts with electrical contact spray and bent the arm ever so slightly to make contact with a new area of the carbon rail

              Comment

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