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Thread: 1991 E32 750il with engine vibrations

  1. #1
    Join Date
    Mar 2008
    Location
    reno
    Posts
    304
    My Cars
    1989 325i

    1991 E32 750il with engine vibrations

    Hi, I've been trying to solve this problem for a while but can't figure it out. When in park or neutral if I rev or raise rpms to about 1500rpms the engine has a pretty significant vibration, very similar to a broken fan blade.

    I've gone through the cooling system, including new fan and fan clutch, all the rear coolant sensors are new. All new spark plugs, distributor caps and rotors as well as checked for vacuum leaks. The car only has 70k miles and motor and trans mounts appear to be in very good condition.

    When driving you don't really notice the vibration unless you are going slow in a parking lot and the engine is around 1300-1500 RPMs.

    I have a few other v12 BMWs and they are smooth as silk. This car has never thrown a check engine light or eml and gets around 15mpg average.

    I'm thinking maybe the DK motors aren't sycned and motor is running different between banks. Should I remove and clean them possibly, I'm not too familiar with what's needed to do that.

    19psi turbo E30
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  2. #2
    Join Date
    Mar 2013
    Location
    Green Bay, WI
    Posts
    1,332
    My Cars
    1992 750il / 1991 560 SE
    Even a difference in banks should not cause an imbalance. Even when the M70 is running on one bank, its shockingly smooth. As such, you may be able to isolate the issue by killing one bank at a time and gauge results. Disconnecting the MAF will kill a bank. Its a quick and dirty test. See if there is a noticeable difference in sides. If you have a performance issue that seems much greater on one side, you can narrow things down a lot.

    From what you describe, it seems more like a fuel supply/misfire issue though. How new are the spark wires? At that milage I am guessing original, but they do age. How are the O2 sensors? I would also check the crank sensors and pick-up sensors for proper resistance. I dont have the numbers handy, Shogun probably does.

    FYI, you probably know that the M70 has two fuel pumps. But if you start leaning towards a fuel issue you can do some fancy plumbing work and operate on one pump at a time to test both out. Or ideally, install some ports to test pressure. Likewise if you suspect is a single misfire issue, you can unplug injectors one at a time and see if there is a change or not.
    ______________________________
    1992 750iL

  3. #3
    Join Date
    Jan 2005
    Location
    Japan
    Posts
    54,654
    My Cars
    11/88 E32 750iL+98 E36M3
    ignition system, donuts, CPS testing data I posted here for the M70 https://www.bimmerforums.com/forum/s...and-M73-engine
    Ever changed the engine mounts?
    DK cleaning http://www.mwrench.com/Whitepapers/DKMotorcleanup.pdf
    Shogun tricks and tips for the E32 series are HERE!

  4. #4
    Join Date
    Mar 2008
    Location
    reno
    Posts
    304
    My Cars
    1989 325i
    So engine has the same vibration regardless if I have a coil or maf unplugged. So whether it's running on the left 6 or right 6 there is no change in vibration. I unhooked each spark wire and also tested again unhooking each injector while running to see if it was possibly not running on all 12 but each cylinder made a difference.

  5. #5
    Join Date
    Jan 2005
    Location
    Japan
    Posts
    54,654
    My Cars
    11/88 E32 750iL+98 E36M3
    valve timing? https://www.bimmerforums.com/forum/s...2-Valve-Timing
    of course, just guessing, but might need to be checked.
    Shogun tricks and tips for the E32 series are HERE!

  6. #6
    Join Date
    Mar 2013
    Location
    Green Bay, WI
    Posts
    1,332
    My Cars
    1992 750il / 1991 560 SE
    ^ That why my next thought. I don't think chain jumps are common, but it should be checked. Also the engine is in interference engine, so if the timing goes too far you end up with very bent valves. Also I would start checking things like the harmonic balancer and fan. I would even pull the viscous fan itself quick. For teh balancer, I think BMW drills them and does not put weights on. I don't recall. If they use weights I have seen instances were a weight fell off causing an annoying vibration. Obviously look over the rest of the belt system too. Basically before opening it up I would do a really intense look-over.
    ______________________________
    1992 750iL

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