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Thread: Low speed 'bucking' question

  1. #1
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    Low speed 'bucking' question

    I've searched for this a little bit, but haven't really found anything real specific to what I'm experiencing.

    1990 5-speed convertible, ~140K miles

    First off, the bucking seems to come from the rear, I'm pretty sure it isn't engine bucking (which incidentally is mostly what I find when trying to research). I could be wrong on this and can be convinced otherwise.
    The bucking is more a horizontal, front to back feeling than a vertical, up and down feeling like you might expect with engine bucking
    More prevalent right after first start up of the day when everything is cold.
    At low speeds/gears, when the throttle is applied, even gently, then released, the car will sometimes start bucking. Not always though.
    Letting off the throttle abruptly is more likely to initiate the bucking.
    Gentle braking will stabilize the car and stop the bucking.

    My theory is that it could be worn differential mounts/bushings?

    Notable facts:
    All front suspension bushings and components are only months old (except shocks)
    Rear trailing arm bushings replaced last 2 years
    Rear wheel bearings replaced last 2 years
    Rear CV joints inspected and re-packed with grease last 2 years
    Trans mount has been replaced last 2 years
    Drive shaft CSB has been replaced last 2 years
    The symptom was happening before all of that was replaced.

    Any ideas appreciated!
    Last edited by sienayr; 11-29-2018 at 02:08 PM.

  2. #2
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    sounds like the rear subframe bushings are gone

    Sent from my SM-G892A using Tapatalk

  3. #3
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    Thanks for reading through my wall of text, that makes total sense and that probably should have occurred to me, I haven't changed those either.

    I seem to remember seeing a procedure that allowed you to drop the subframe down enough to push the bushings out by getting a cap on top that a threaded rod goes through, without having to drop the whole subframe. My 540i wagon was like that, with the special tool I did all four subframe bushings in about two hours by just loosening the bolts and letting the subframe drop down enough to get the cap of the tool on top to push out and push new in.

  4. #4
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    No e30s, again :(
    that definitely is easier then dropping the whole assembly and trying to pound those stupid things out with everything you reach. I had to burn mine out with a map gas torch
    No e30s again.

  5. #5
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    I didn't see where you mentioned replacing the (front) motor mounts ... that would have been the first thing I checked. But I'm not an experienced E30 guy like some here are.

    Maybe apply the emergency brake hard, and place some wheel stops behind and in front of the wheels ... then GENTLY slide the clutch (NOT fully engaged!) in first gear, with the engine at idle, while someone is watching (from a SAFE distance) the rear subframe (and / or) the front motor mounts.

    You should be able to see any unusual movement.

  6. #6
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    If the diff mount is that loose, I think you could tell by looking at it. If the subframe bushings were that loose, I would think that rolling the car from forward to reverse and back, using the clutch to load the driveline each direction, you would be able to make the subframe/diff clunk around. Another idea might be to put the car on 4 jack stands, start the engine and run through the gears, again going from forward to reverse to load the driveline. Using the e brake would def help, yes. Just make sure you don’t put the rear jack stands under the subframe, of course.

  7. #7
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    Thanks for the tips all. I will try out some of the ideas here and see where I get. Either way planning to change the sub frame bushings, I'm sure motor mounts need it too.

  8. #8
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    I replaced the hot air intake that has been on my car since purchase (you know, the K&N air filter sticking out in the engine bay sucking hot air right off the engine) with a stock air box I picked up from the yard yesterday, and it almost seems like it has helped this symptom. The car doesn't seem to want to start bucking as bad when I let off the throttle. It's still there, just seems less jerky than before.

  9. #9
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    A follow up, had the car up in the air and noticed there is play in the differential, can move one tire back and forth while the other tire doesn't move. It also sounds a lot like the 'bucking', and it makes sense that play like this would cause the symptoms I have. (Got the motor mounts changed since the post, but not the sub-frame bushings.)

    Is it normal or abnormal to see this much play in the differential? It's an open diff. Thanks for your time.

    https://1drv.ms/v/s!AngWLA3-5pQzgzW9...82W8U?e=6O3dCK

  10. #10
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    No e30s, again :(
    I notice standard shift vehicles get more bucking, just from being standards. I don't know why but I think the weight of the vehicle versus the engine
    No e30s again.

  11. #11
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    I have that too especially in reverse. I watched a YouTube video where a guy demonstrated it in his dodge viper. He indicated his quibo might be worn a bit making it worse, but explained it was kind of a normal thing for manuals, especially if old with worn parts. I'm sure it could be made worse by loose or worn parts. I myself may have a worn rear quibo but havent take agood look. I just either add more gas or re fandangle the clutch. I'm a noob to manuals though and haven't really experienced it on modern manual cars I drove.

  12. #12
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    Generally speaking should there be play in the diff like in the video? Or when you push one of the tires should there be no play, and the other tire moves? The motion or 'play' seen in the video was not moving the other tire.


    https://1drv.ms/v/s!AngWLA3-5pQzgzW9...82W8U?e=6O3dCK
    Last edited by sienayr; 06-15-2020 at 10:56 AM.

  13. #13
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    Quote Originally Posted by sienayr View Post
    Generally speaking should there be play in the diff like in the video? Or when you push one of the tires should there be no play, and the other tire moves? The motion or 'play' seen in the video was not moving the other tire.


    https://1drv.ms/v/s!AngWLA3-5pQzgzW9...82W8U?e=6O3dCK
    That play is a bit excessive to me, we just had my own E30 up on the stands and we checked all 4 corners for play. There is very minor play on my E30 with factory LSD, maybe just 1mm or even less of play before the driveshafts all move simultaneously, one has to have a very good eye to notice that tiny play.

    Double check the torque on the axle to hub nut, i believe they are 190nm but they also have that securing washer that needs to get "bent" to lock the nut, would not hurt to check. If everything seems okay..i would guess that there is play in the rear drive flange itself maybe? (where the bearing presses into). There are grooves or "teeth" in there, to match up with the driveshaft, maybe they are a bit ground down?

  14. #14
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    Reading your thread got me more concerned about my own. I have an ix, but I have bucking in reverse, which I can eliminate by moderating the gas/clutch, but I also have a clicking when I back I to the driveway. Sounds rotational. Would that be more of a rear driveshaft can issue maybe? I have not ever inspected the rear driveshaft on this car 🤪. No noises or bucking when creeping forward.

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