cmpcpro

Hey all, I've been so busy with my business I haven't been on in ages but I have had this noise for months now.. It's like something is loose in the back. I can only hear it if I am going over certain bumps, usually in an alley or something when it's like a bump being hit a certain way. I want to say a bump that makes the car sway a little but I may be completely off. I have looked under the car, and the endlinks arms etc look good.. I am not sure if it's the air struts going bad, or what.. it almost sounds like something in the trunk, but I have checked that too.. almost like the trunk is rattling/hitting the car, but obviously thats not happening.. Is there anything common that makes noises in the back of these cars?

Any ideas would be appreciate! Thanks!

Welwynnick

I've got something similar, which is quite difficult to describe, but you know something's wrong. I've had four W220's, and the latest has a subtle deficiency in its ride & handling. The rear end is always bumping, rumbling and weaving around on bumpy surfaces, but the bumping doesn't correlate with the bumps in the road. There's something else going on there, something moving around with a mind of its own. I took the wheels off and levered all the suspension arms, but I found no loose joints or bushes. So I assume I have a worn subframe mount or two. There's not much discussion of this , but I did find a couple of useful posts. You'd expect subframe mounts to be difficult, but they sound feasible for an experienced DIY owner.

http://forums.mercedesclub.org.uk/sh...ad.php?t=90826
http://forums.mercedesclub.org.uk/sh...ad.php?t=90827

Nick

cmpcpro

Hmm.. thanks, I will take a look at that! I was def. thinking something like the subframe.. because it started out like a bass-ie thud, and it seems like this could be it getting worse into now a little bit of movement. The links you left though are for the front subframe and I haven't found any for the back, but I will keep searching.

Welwynnick

The links talk about changing the front mounts on the rear subframe. Changing subframe mounts is something that I'm wary of, but those accounts suggest its feasible without necessarily having special tools.

Nick

biker349

i had this problem. turns out the spare tire was loose.

cmpcpro

I am pretty sure I checked the tire and it was tight but I didn't check the jack I believe.. and it could surely be a jack sound.. I guess I will have to take my whole system back out of the trunk and check it all out.. I really have a bad feeling it's a failing Airmatic strut, but I really hope not..

Welwynnick

One day I'll get to take a look at my subframe mounts, but in the meantime I had an idea. My car is plagued with misfires, and I've replaced the coil packs, MAP sensors and O2 sensors. On the whole, its running great, but sometimes it throws a wobbly even when running gently, not just when gunning it.

Sometime you hear about obscure causes of faults, and I've thought about another possibility. When mine used to misfire, it often felt it was running over a rough surface, with lots of fine, regular bumps. And when I drive over such surfaces, more than once I mistook the road induced vibration as a misfire. What I take to be worn mountings seems to cause an amplification of certain small vibrations. I presume that the whole subframe is shaking backwards and forwards on its worn mounts.

Now, I read that modern ECUs monitor engine speed very closely indeed. If one cylinder fails to fire just once, the engine will slow down slightly - even a V12. That variation is detected, and if it recurs more than so many times, its taken as a misfire. Since that will cause exhaust emissions to increase, its flagged as a fault, and in bad cases will put the engine into limp-home mode.

Since rough roads cause the wheels to speed up and slow down over bumps, this could be seen as a misfire when there isn't one, so the ECU has a "rough road" inhibit that makes it ignore uneven running. Its all very clever. However, the inhibit threshold is presumably set during development using cars in a normal condition. Mine isn't normal, and seems to exaggerate certain types of bumps, setting up an oscillation/vibration in the subframe and drivetrain.

My new theory is that the ECU detects a cyclical engine speed variation on what it takes as an otherwise "smooth road" and mistakes it for a misfire. What do you think? I'm not sure I've explained this very well, but i'm definitely going to look into repairing those subframe bushes, adn see if that makes any difference.

nick

cmpcpro

I would be extremely surprised if that was the cause of your misfire problem.. but nice dd.

AH1W-COBRA

if you are referring to the thud, clunk, that occurs when the rear struts top out than i have to assume it is normal. mine will only do it when the suspension is unweighted. like driving through a parking lot and hitting a speedbump slightly to fast and the suspension "falls away" on the backside of the bump and voila, thud, clunk. ther rebound dampening on the airmatics isnt stiff enough to overcome the pressurized strut unweighted. just my two cents.

Welwynnick

I had a look at the subframe mounts last weekend, but they didn't look too bad.

However tonight I unwittingly found a clue about what makes the S600 tick. I went out to fill the tank tonight (I had one mile to empty!) and I got a puncture on the right rear on the way back. The spare is a 245/45/18, same as the front, but smaller than the 265/40/18 rear.

Coming home with one 265/40, instead of two, was a very illuminating experience. The ride was suddenly smoother and quieter, and my car was closer to the magic carpet that my old N/A S600 used to be. That car had 245/45's all round, and had the most wonderful chassis - a prefect blend of grip, balance, agility, stablity, ride and refinement. The V12TT is definitely inferior - noisy and harsh - but I guessed the subframe bushes might be the guilty culprit. Not so, based on my experience tonight.

Nick

L@YL0W

It might be your Airmatic Rear axle level sensor that hits in your axles when you go through those bumps.... I dont know when they fail, or why they fail.... But i had my car lowered to the floor with the airmatic off for over a year and nothing happened.... Untill i garaged my car for over two months and decided to take it out again.... I had to remove the sensor cause mine was hitting it constantly, plus i do have my airmatic off so i dont really need to replace it.... The axle level sensors exact location is on top of the rear left axle by the differential... Hope this helps

Fermin Grajales

Checking my spare tire now!