Weird rear wobble

[estoguy]

Hey guys... need some input.

I've got a weird rear wobble... only happens when I'm decelerating with light pressure on the accelerator. Feels like something is loose in rear, but no loud clunking, more like a wubba sound (not sure what else to call it - almost like there is a tub of water sloshing around in the trunk).

If I add throttle or take my foot off the accelerator entirely (and the fuel economy meter drops to zero), it goes away. The only other time I can replicate it is by turning the car off and letting it freewheel... that wubba-ing happens around 50-60 km/h (roughly 30-40 MPH). If I hit the brakes and slow down further, it goes away below 50.

If I'm traveling with good steady accelerator pressure (manually or cruise control) or accelerating, no problems.

I've been doing a lot of googling on this... my research seems to be pointing towards rear shock mounts or possibly looseness in the RTAB bracket.

I did my RTABs about 2 years ago, about 120K km ago. I figure I got them tight (didn't have this issue after doing the job), of course, I was guestimating, as getting a torque wrench on those bolts is damn near impossible. As I recall the torque for those is 65 ft/lbs?

Rear shock mounts were done slightly after that. I used the Meyle HD mounts, with the E46 part number.

Thanks!

[dnguyen1963]

With the kind of mileage you have on your car, something ought to get loose. Are you sure the fuel tank is secured? How are your tires? Unevenly worn tires can make all kinds of strange noises and only under certain speeds.

[estoguy]

I don't think there is anything going on with the subframe bushings... no clunk like I had before their replacement. Almost like I'm pulling something behind me and it's moving around.

I hadn't thought of the fuel tank, but I'll check it.

Tires are good... they were almost new when I got them last year, and there hasn't been any weird wear I can detect.

[estoguy]

Took some video, both standing still and with car in motion.

https://www.youtube.com/watch?v=Qo6hycmBB8Y

https://www.youtube.com/watch?v=os7Bm51J7mQ

[dnguyen1963]

That sounds like something is loose somewhere in the suspension. Take a look at your rear shock mounts again to see if there is a small crack. The other suggestion I have is to have a friend push the car up and down and you can try to find where the noise is coming from with a stethoscope. Good luck.

[estoguy]

Thanks for the input... yeah, I'm really leaning towards the rear shock mounts. When I push down on the rear, things feel pretty solid and firm, except for that clunking. I'm away from home, so can't jack the car up until tomorrow.

I've taken a look at the subframe bushings, and they look OK. From my video, I don't seem to be getting any weird side to side movement on the wheels.

For around $60 and less than an hour's work, I'm willing to try the rear shock mounts and go from there.

If anyone else has any insights, please let me know :)

[jlevie]

Throwing parts at a problem and hoping for a favorable outcome is not the best way to go.

You have the same thread going on BFC and I have posted a diagnostic there that will reveal exactly what is wrong.

[estoguy]

I didn't see your post over there until after I'd posted last here... gonna get a good look when I'm home tomorrow. :)

I usually post on both forums, because there are people who frequent one, but not the other who are knowledgeable in all things E30.

You're one of the few who goes between the two. :)

[estoguy]

Well, I did do the rear shock mounts and there has been some improvement. Still rattly though. When I had her jacked up, I did try moving the trailing arm, and seemed to get some movement... not really sure if it's more than it should be. When I did them about 120K km back, as far as I knew, they were original. The subframe mounts looked ok, but I was having trouble getting a look.

Based on Jim's earlier post, when I get it up on the ramps, what are some things I should be checking? Also procedure as to how to proceed would be helpful. Thanks guys!

[estoguy]

I think I tracked down my issue. Finally got under the car... found that the outer RTAB bolts were on the loose side. Tightened them up some more and took her for a drive. All those sounds from the video are gone. Thanks again Jim and others.