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Thread: Need help diagnosing bad engine vibration

  1. #1
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    Need help diagnosing bad engine vibration

    98 M3 track car. The car has a bad intermittent vibration between 3000-3200 rpms. It mostly happens on deceleration while the vehicle is in gear, although I've replicated it while the clutch is depressed and rpms held steady, therefore I believe it is somewhere within the engine. Since I've owned the vehicle, I have always felt this vibration, but I've since installed poly engine/trans mounts and the vibration is really noticeable now and it worries me. Yes, the poly mounts are installed and torqued correctly. This vibration is way beyond normal NVH.

    FYI, the driveshaft, guibo, engine/trans mounts are all new. Wheels are balanced, diff works fine. I think I've narrowed it down to the engine, but I can't pinpoint it. I've tried checking the pulleys for play and the only one that moves is the a/c pulley since the clutch is not engaged when the vehicle is off. Any guidance or help?
    Last edited by timsev; 09-27-2017 at 04:26 PM.

  2. #2
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    Quote Originally Posted by timsev View Post
    I've tried checking the pulleys for play and the only one that moves is the power steering pulley since the clutch is not engaged when the vehicle is off. Any guidance or help?
    Power steering doesn't have a clutch, you mean A/C?
    Caprica Junkie

  3. #3
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    Quote Originally Posted by boarder2k7 View Post
    Power steering doesn't have a clutch, you mean A/C?
    aye, edited original post.

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    Have you checked all the hoses? Vacuum leak maybe?
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  5. #5
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    Quote Originally Posted by Pelican Parts View Post
    Have you checked all the hoses? Vacuum leak maybe?
    Yes, all hoses and vacuum lines are like new and recently replaced during engine refreshing. Should I be looking at the crank damper or flywheel? It feels so specific in the RPM band.
    Last edited by timsev; 09-27-2017 at 05:52 PM.

  6. #6
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    Is it the VANOS unit making that vibration? Is it broken motor mount bad or is it a high frequency vibration/noise?

  7. #7
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    Poly transmission mounts amplify engine vibration more than anything - particularly if they're a straight bolt-through design. A lightweight flywheel may also give some resonance at different RPMs. You didn't mention what type of flywheel or the type of mounts you have.

    When I had a dual mass flywheel and stock clutch I could get some big vibrations in the 3000 RPM range. I found that trans mount material had a significant effect on what I felt. With stock rubber mounts the vibration was barely noticeable. With UCC red mounts, which are an isolated poly design (fastener doesn't pass all the way through the urethane material), the vibration was more noticeable but tolerable. With AKG 75D mounts (straight bolt-through design) the vibration was gnarly. So I ended up using UUC reds (which are now UUC blues) as the compromise for that setup.

    After changing to a Tilton flywheel/clutch the vibrations are pretty much gone. This is just a theory of mine, but I think in some cases the two masses of the DMF would get "out of phase" in a way that increased vibration. I felt this because the vibration level was different when I approached the problem RPMs from acceleration or deceleration. Deceleration would get things vibrating... it was most noticeable in steady-state cornering through sweepers. I found with some experimentation that if I stabbed the throttle quickly (to load the DMF in the other direction) the vibration would calm down. And that was on a DMF that I purchased brand new. The real test is this weekend when I combine the Tilton setup + bolt-through AKG mounts and hit Thunderhill. If things stay smooth with the previously intolerable AKG mounts then I know it was something out of balance with the previous flywheel and clutch setup.

    Another thing that smoothed out vibration was replacing the driveshaft centering bushing. It's the pilot bushing that's pressed into the guibo end of the driveshaft. Of course that's a vibration that would be related to vehicle speed and not engine RPM.

  8. #8
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    Quote Originally Posted by NoLastName View Post
    Poly transmission mounts amplify engine vibration more than anything - particularly if they're a straight bolt-through design. A lightweight flywheel may also give some resonance at different RPMs. You didn't mention what type of flywheel or the type of mounts you have.

    When I had a dual mass flywheel and stock clutch I could get some big vibrations in the 3000 RPM range. I found that trans mount material had a significant effect on what I felt. With stock rubber mounts the vibration was barely noticeable. With UCC red mounts, which are an isolated poly design (fastener doesn't pass all the way through the urethane material), the vibration was more noticeable but tolerable. With AKG 75D mounts (straight bolt-through design) the vibration was gnarly. So I ended up using UUC reds (which are now UUC blues) as the compromise for that setup.

    After changing to a Tilton flywheel/clutch the vibrations are pretty much gone. This is just a theory of mine, but I think in some cases the two masses of the DMF would get "out of phase" in a way that increased vibration. I felt this because the vibration level was different when I approached the problem RPMs from acceleration or deceleration. Deceleration would get things vibrating... it was most noticeable in steady-state cornering through sweepers. I found with some experimentation that if I stabbed the throttle quickly (to load the DMF in the other direction) the vibration would calm down. And that was on a DMF that I purchased brand new. The real test is this weekend when I combine the Tilton setup + bolt-through AKG mounts and hit Thunderhill. If things stay smooth with the previously intolerable AKG mounts then I know it was something out of balance with the previous flywheel and clutch setup.

    Another thing that smoothed out vibration was replacing the driveshaft centering bushing. It's the pilot bushing that's pressed into the guibo end of the driveshaft. Of course that's a vibration that would be related to vehicle speed and not engine RPM.
    Thank you, this is some great insight and your issue sounds very similar. YES I have the original, oem dm flywheel/clutch. When the car was completely stock, I would notice the vibration, but it was tolerable and it was ALMOST non-existent. After I stripped the interior, I noticed it more, but still tolerable. Then I went with Revshift 75d engine and trans mounts, and it feels like the car is going to buzz/rattle itself apart. I know it's not normal. This progression shows me that the stiffer mounts aren't the cause of the vibration, but only intensify it.

    Your experience mirrors mine when I was at Heartland Park this year. Long left hand sweeper, very slightly decelerating, triggering the crazy vibration. Giving more throttle would alleviate the problem, at least until the next lap. During normal driving it seems to be intermittent. I thought I was going crazy until one time I held the throttle steady and clutched in, and the vibration continued, so I was able to narrow it down to the clutch forward. I'm hoping you're right and I can just step up to a single mass flywheel and be done with this.

    If anyone else has any similar experiences or advice, please chime in, but "NoLastName" has been tremendously helpful.

  9. #9
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    Update for timsev: I drove on track today with the AKG 75D transmission mounts and Tilton clutch. My hunch was correct. There was a lot more gearbox noise and NVH overall but no 3000 RPM decel vibration like before. To me that kinda indicates the previous intermittent bad vibration was related to the DMF and stock clutch assembly. Maybe DMFs just aren't super happy when used under track conditions... I dunno.

  10. #10
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    Quote Originally Posted by NoLastName View Post
    Update for timsev: I drove on track today with the AKG 75D transmission mounts and Tilton clutch. My hunch was correct. There was a lot more gearbox noise and NVH overall but no 3000 RPM decel vibration like before. To me that kinda indicates the previous intermittent bad vibration was related to the DMF and stock clutch assembly. Maybe DMFs just aren't super happy when used under track conditions... I dunno.
    Thanks for the update. I hate just changing parts to eliminate the problem, especially considering the cost of a flywheel/clutch, but this is the only thing I can think of that would create such an issue. Now I've got to decide which flywheel/clutch combo to go with. Tilton is too much for me. Not sure if I'll go JB Racing or the ECS one looks good too.

  11. #11
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    Attempting the reawaken this thread... a couple months ago I replaced my oem dual mass and clutch kit (180k) with an fx stage 2 single mass 14lb kit recommended by a friend with the same engine/trans I have. (M52/zf 5 speed) also had the assembly balanced at the local Napa machine shop to within “race” specifications. I immediately noticed an aggressive vibration coming through the chassis. I have condor SOFT poly bushings which do have a pass through bolt. I’ve changed engine mounts and I had a well known and respected shop take a look. The tech didn’t even have to drive the car just revved it and said “flywheel” The vibration happens with clutch in and out but goes away after 2k rpm. Highway driving feels very smooth and “normal” My question is do I bite the bullet and order a new flywheel? I’ve driven 200+ miles with this setup to see if it is related to clutch break in process but nothing has changed. When I downshift to third an increased vibration can be felt through the chassis and in the shift knob. It isn’t spark/fuel/maf/vacuum related I’ve tested all these theories. My thought is maybe a more stock weight single mass? I’m not really interested in going back to a dual mass as it is a track car in the making. I may have gotten a random dud flywheel so any advice is welcome. Thank you

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