Vibration during acceleration at specific speeds

[Jeff]

Hello, first post on here, so I hope this is the correct section.

I’m having an issue with vibration under acceleration at specific speeds (45-60) and right at 70, everywhere else is fine. My car is rwd and manual. The vibration happens regardless of what gear I’m in. The more throttle I apply the worse the vibration is at these speeds, but when coasting (clutch in) or decelerating via engine braking the vibration does not occur. I noticed it several weeks back and it seems to be getting progressively worse.

The wheels are fairly new, but to be safe I had them re-balanced and rotated, I’ve inspected the u-joints and didnt notice any play, and I’ve replaced the front wheel bearings. (havent had time to check the rears yet, but the fronts were pretty bad and replacing them has made no difference).

Things that I know are worn, but I dont think are the culprit are the engine mounts, idler arm, and one of the rear brakes is dragging slightly.

I think I’ve covered most of the basics, and maybe I’m overlooking something. I have plans already to replace the known worn parts, but my main concern at the moment is whatever may be causing this if it isnt one of the things I already plan to replace. Any ideas? Thanks.

Topic

[Alexander B]

The vibration being independent of which gear excludes the engine and pretty much the gearbox too.

Does the prop shaft have a center bearing? check that for play. Especially if its got like rubber vibration dampening stuff and its all perished.
(Apparently a common problem with BMW’s, among others…)

[Jeff]

It doesnt have a center bearing

[Alexander B]

Well, it it doesn’t have it, it can’t be bad, right?

I guess I’d be looking at other rubber bushings and things in the rear suspension, but I really don’t know RWD cars that well.
Does it have one of those rubber drive disks? Or just the U joints?

[Jeff]

Not quite sure what you mean by rubber drive disks. From what I can tell it only has u joints.

[Alexander B]

Again, if it doesn’t have it, it can’t be broken.

[Jeff]

nope, doesn’t have anything resembling that.

[Jeff]

I’ve finally managed to fix it. I’m putting this here incase anyone in the future has the same issue and finds this thread.

I ended up replacing the idler arm and the engine / trans mounts to no avail.

Turns out it was both the drive shaft and the U-joints. The u-joints were bad. One had a cracked cap and no bearings left at all. So I replaced both and re-installed the driveshaft. This changed where and when the vibration happened, but didn’t eliminate it.
Under the assumption that I may have lost a weight and it was out of balance, I took it to a balancing shop who pointed out that the caps were spinning in the yoke, and that I either needed a new shaft entirely, or I could try to find someone who could weld on a new yoke and then have it balanced.
I found a good deal on a new driveshaft online, so I ordered that rather than go through the hassle of re-fabrication/balancing.

The vibration is now gone, and with the other things I did trying to track down the problem, its now the smoothest its been since I got it.

Long story short. Just cuz you dont want it to be the drive shaft thats wrong, doesnt mean the driveshaft isnt wrong.

[Author]

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