BMW 320CD 2006 knocking issue

[Djnikon]

Ok, I'm stumped by my BM and need some fresh thinking.
When driving, if I hit a bump or even slight pothole on the rear passenger side, the car bounces violently, shuddering almost and loses traction on that side momentarily. There is also a knock noise that comes from it when this happens.
Stationary if you bounce the car there is no noise what-so-ever and the compress and return rate seems normal enough.
I stripped out the shockie and mount at the weekend and found to my surprise that there is nothing wrong with the shock, no misting of the oil or anything sinister. The top mount looked worn so I thought best to replace. Still the problem exists. Bushes look fine, spring is fine and I can't see anything wrong with the shockie (and at £118 a pop I'm loathe to replace if no fault).
Car is a 2006 (55 plate) E46 2.0 Diesel Coupe.

Anyone have any ideas?

Cheers

 

[jonnyfishing]

A shocker will react differently with you pressing down on it and the weight and momentum of the car. The shocker is there to stop the car bouncing on the spring, so I would go for new shockers or get a garage to give their opinion.
They should be changed in pairs.

 

[Djnikon]

Thanks Johnny. I realise that pressing it won't mirror the road conditions, however in my experience when a shockie has gone it makes a noise even when stationary. A garage would only do the same checks i have done in terms of checking for oil misting and compress/return rate.
The pair on this at the minute are only 4months old (genuine M-Tech Bavaria numbers) and it doesn't show any signs of defect - hence why I'm stumped.
The shockie is there to allow the car to bounce on the spring without "shock" or "dull drop" as if it was to stop it bouncing on the spring, why would you have a spring?

Thanks for your reply anyhoo

 

[ray99]

Have you checked the springs properly? They always break on the E46, usually the bottom coil but sometimes the top one as well.
Easy to see the bottom coil with the wheel off or with the car jacked up run your hand around the coils and before you get to the bottom you will find a jagged edge.
Not unusual to replace E46 rear springs every couple of years. (Same applies to Z4 and X3)

Edit. If nothing visible switch off the DSC (press and hold the switch for at least 3 seconds) and try driving it over a few bumps which normally cause your problem.

 

[nickso]

A well known E46 problem, but possibly a bit late for your model year, is the subframe mounting point on the boot floor cracking, allowing the subframe to move independantly of the body. Not sure if it will act like yours is and I'm sure they had it sorted by your year but perhaps worth a look as you seem to have checked everything else.

 

[Djnikon]

Thanks nickso, I will get this checked. Had both shockies off last night and put them both through compression tests. Same result (to within +/- 1lb each way) on each test.
Subframe better not have cracked but if it has then at least it answers the question.

 

[Belle427]

if it bounces violently as you say it has to be severe wear in a bush like component, i cant see shock or spring problems doing this.

 

[gblades]

What about the trailing arm bushes. They used to wear out fairly frequently on the E36

 

[Djnikon]

Finally got to the bottom of this after stripping the whole suspension out. The road spring had a crack in it that was almost invisible from the angles that you could see it from. Replaced the spring and joy of joys it's back to it's good old self.
Still Shiite in bad weather but hey, its just a big German sledge in the snow. I still love it.

 

[ray99]

See post 4. What took you so long? Spring takes seconds to remove and not much longer to refit.
When the spring breaks there is usually very little difference in the ride height but the spring rate changes dramatically and results in much more suspension movement and less wheel control.
Best way to check is by feel. With the wheel fitted it is not possible to see the bottom of the spring but if you run your hand around the coils you can always feel the rough edge where the break is.
Glad you got it sorted.

 

[nickso]

See post 4. What took you so long? Spring takes seconds to remove and not much longer to refit.

Maybe he is like me? Take one thing out and then just do everything else that looks ropey too.