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Thread: bucking like a bronco under hard acceleration

  1. #1
    Join Date
    Mar 2015
    Location
    Australia
    Posts
    7
    My Cars
    1994 BMW 540i

    bucking like a bronco under hard acceleration

    I have a 1994 540i automatic. It runs perfectly at idle and under normal acceleration but if I put the accelerator down all the way the revs start jumping between around 3,000 and 3,500 cycling about twice per second and the jerking is quite violent. It does not seem to matter if the engine has been running for some time or only just warmed up. Mechanic thought it might be the flow sensor on the air intake and replaced it but this made no difference.

    Any ideas?

  2. #2
    Join Date
    Jun 2009
    Location
    McKean township Pennsylva
    Posts
    10,542
    My Cars
    1999 M3 , 1999 328is
    Is it able to exceed 3,500 rpm? IS this the only band of speeds that it bucks?
    Current fleet:
    1999 BMW e36 M3
    1999 BMW e36 328is with rotary valve engine head
    1999 Oldsmobile Eighty Eight
    1990 Jeep Comanche Eliminator
    1962 Austin Healey Sprite

  3. #3
    Join Date
    Mar 2015
    Location
    Australia
    Posts
    7
    My Cars
    1994 BMW 540i
    If I don't plant the foot yes, revs will go right up to redline. In normal driving it is fine. It is only when I try to accelerate maximally that the bucking occurs and even then not everytime.

  4. #4
    Join Date
    Oct 2014
    Location
    Loma Linda, CA
    Posts
    159
    My Cars
    1998 BMW M3 Coupe
    Are there any CELs on the dash? If so, it may be helpful to pull them. So far it sounds like coils to me
    primum non nocere

  5. #5
    Join Date
    Jun 2009
    Location
    McKean township Pennsylva
    Posts
    10,542
    My Cars
    1999 M3 , 1999 328is
    This one has me pretty stumped. I have not experienced such a thing in any vehicle.

    It sounds like some weird fuel issue to me. Is there a possibility that your fuel filter is clogged or you fuel pump is dieing?
    Current fleet:
    1999 BMW e36 M3
    1999 BMW e36 328is with rotary valve engine head
    1999 Oldsmobile Eighty Eight
    1990 Jeep Comanche Eliminator
    1962 Austin Healey Sprite

  6. #6
    Join Date
    Mar 2015
    Location
    Australia
    Posts
    7
    My Cars
    1994 BMW 540i
    No CELs on the dashboard :-(

    This is a tough one and certainly has stumped my usual mechanic.

  7. #7
    Join Date
    Jan 2008
    Location
    Arida Zona
    Posts
    30,107
    My Cars
    z3
    MAF, plugs, coils would be the first things I look at

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  8. #8
    Join Date
    Oct 2014
    Location
    Loma Linda, CA
    Posts
    159
    My Cars
    1998 BMW M3 Coupe
    Quote Originally Posted by BimmerBreaker View Post
    MAF, plugs, coils would be the first things I look at
    This and check fuel pump pressure, perhaps replace fuel filter
    primum non nocere

  9. #9
    Join Date
    Mar 2015
    Location
    Australia
    Posts
    7
    My Cars
    1994 BMW 540i
    Thanks. Good suggestions. Can I ask what MAF is?

  10. #10
    Join Date
    Oct 2014
    Location
    Loma Linda, CA
    Posts
    159
    My Cars
    1998 BMW M3 Coupe
    MAF is Mass Air Flow sensor. Every fuel injected car has it and it is located in the intake piping prior to throttle body. It basically calculates amount of air flying by and communicates this information to the ECU.
    primum non nocere

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