Diff mounting bush

Post Wed Jan 18, 2006 8:16 pm

Hi,

First time on here for a while - I hope someone can help (and I can remember how to use this board properly!)

My 320i cabrio has an annoying rattle/clonk from the rear end/rear seat area - most noticeable when power on/off at low speeds or going over a bad road surface. Does it when coasting too so I reckon it cant be the diff itself. I have changed the shock top mountings and the anti roll bar drop links. No improvement. Could it be the diff mounting? Any thoughts/advice appreciated (yes I have tried taking evrything out of the boot!) I think the bush is the original as I have most receipts from when the car was new. It's done 49000 miles but is 16 years old of course. The bush itself looks dodgy to me but then I dont know what they looked like when new.

Any wisdom appreciated before I spend another weekend under the car rather than driving it.

I have heard that replacing it with the solid bush from a z3 3.0 or M roadster is a worthwhile mod anyway. Anyone tried that?

Thanks for your help.

Post Wed Jan 18, 2006 8:53 pm

what about the prop centre mounting? just done mine, was getting a knocking from under the rear seats area.the bearing casing had snapped of the mounting.should be easy enough if it is this.

Post Fri Jan 20, 2006 3:54 pm

fuzzy wrote:what about the prop centre mounting? just done mine, was getting a knocking from under the rear seats area.the bearing casing had snapped of the mounting.should be easy enough if it is this.

Thanks mate, I'l check this out. I do occasionally get clutch judder especially when reversing so this could be linked.

Post Fri Jan 20, 2006 4:15 pm

Sounds more like the rear bushes to me if you feel the knocking when your on and off the throttle.

Post Fri Jan 20, 2006 4:41 pm

Martinaston wrote:Sounds more like the rear bushes to me if you feel the knocking when your on and off the throttle.

What are the symptoms of a knackered diff bush then? Getting my rear bushes done but don't really want to do the diff - LSD time soon

Post Fri Jan 20, 2006 5:09 pm

Ziggy wrote:
Martinaston wrote:Sounds more like the rear bushes to me if you feel the knocking when your on and off the throttle.
What are the symptoms of a knackered diff bush then? Getting my rear bushes done but don't really want to do the diff - LSD time soon

That's a good question. From what I gather the diff effectively hangs off the back of the subframe, and the diff mounting takes some of the weight of the diff by holding it up to the boot floor. If the bush is knackered the back of the diff drops down and the subframe flexes with it, being allowed to move by the flex of the 2 mountings that are just inboard of the 2 rear jacking points. I think this can upset the rear wheel camber as well as allowing the diff to move when you put the power on - and I guess it does the subframe mountings no favours either. Anyway it was enough to make me think that the clonk on my car, which seems to happen when I put the power on but also happens when rolling along at low speeds under no power at all, could be caused by the diff mounting. I dont know how to check it though, but I do know I've already changed the anti roll bar drop links and the shock top mounts. But I'm guessing, and I dont like guessing - especially where I might have to spend money.

Post Fri Jan 20, 2006 6:18 pm

If you don't want to spend money then get under the car and have a look, i've never seen a worn diff bush in me life but then if i went to buy a car with a shagged diff bush i'm pretty sure that would be the least of my worries.
The diff bush sitts on the back of the diff and just holds it up against the bodywork, whereas the rear bushes take all the twisting forces created by the drivetrain and have to support the weight of the car.

have a read through this it might help explain it

http://www.e30zone.co.uk/modules.php?na ... ing+points

Post Fri Jan 20, 2006 6:39 pm

Thanks, that other thread is really helpful. I'll try to find time this weekend to get down under so to speak.