Help - My car bounces

I have owned my 1991 420SEL for about 1.5 years. When I purchased the car (from a dealer), I knew that it needed new tires all around. I immediately took it to a trusted and well recommended MBNZ mechanic. He recommended the Michelin MXV4 Plus. While checking out the car, he stated that the upper and lower control arm bushing were shot on the left side and so I had them replaced. The mechanic recommended Winston tires for the tire replacement and alignment and so I had all four tires replaced and the alignment done there. The trouble began when I got the car back. It drives great at speeds under 55+ MPH. But faster than 65 MPH on a not-so-smooth road and it drives like a dump truck - the roughest ride I have ever been in short of an SUV or truck. I have gone back to my regular mechanic and several other recommended mechanics (some that specialize in suspension). I had all 4 shocks replace with Boge Automatics (upon recomendation). I have had the suspension checked from top to bottom, the engine, trans, and differential mounts checked, and the car re-aligned and tires re-balanced too many time to mention. It appears to get a bit better but never the smooth ride expected from a MBNZ of this caliber. I should note that the degree of rough ride varies from day to day and road to road. The bouncing had begun to loosen panels within the car and now some interior parts are ratteling.

Help!!! - My mechanics don't know where to turn and I am tired of spending money on this problem.

A great deal of gratitude would be in order for anyone who could give me any clues.

P.S. - The car has about 98K miles on it.

[Edited by mrbroche on 05-28-2001 at 06:43 PM]

[rdetoy]

First, check to see that the tires are round. A quality tire servce shop, *not* Winston, can check to see if the tires are true and true them if needed.

Second, have someone knowledgeable and capable check your wheels to make certain that they are round and true. It only takes a minor 'curb surfing' episode to throw things out of whack. If your wheels are 'square', they will never balance properly.

Third, have the wheel/tire assemblies balanced while mounted on the car. Do you use locking lug nuts? It doesn't take something that large to throw off the balance and balancing on the car can often compensate for problems that do not show up otherwise.

Finally, have a proper four wheel alignment performed at a MB dealer. It is worth the few extra dollars to have the job done right the first time.

Regards,

Richard,
Thanks for the good advice. When I pondered over you response, I came to realize that not one of these mechanics have checked the "roundness" of the tires or the "trueness" of the rims. They always go straight for the suspension and make suggestions as to what parts(s) they want to replace (even if they have just been reciently replaced).

Is the dealer a good place to have the tires and rims checked or do you think I am better off at an independent tire dealer? The reason I ask is that I immediately went to the MBNZ website and found that one of the dealers in my area is offering a special on balancing/rotation and alignment. $99 for the alignment and $48 for the balancing and rotation (usually $149 and $75). And it just happens to be the only MB dealer out of the three in my area that I have any trust in whatsoever.

Again, thanks for the advise

Brian Roche

[rdetoy]

Those are good prices for those services and I would take them up on them. However, they may not be the place to go for the other services. Here's the thing, if you have the rotation/balance/alignment work done at the dealer and everything is OK, then you're done. If you still have a problem, then you have a reliable baseline of what's been done and maybe they can recommend a tire shop that will be able to do the other diagnostic and fine tuning work, assuming they don't do that sort of thing. A shop that supports/services high performance sports cars and race cars may be able to do the truing work. If the problem is determined to be the wheels, the tires will need to come off for the work to be performed, but the diagnostic work can probably be done at the dealer with a dial indicator gauge. Ask them about it and see what they say. Hope this solves your problems.

Down the road, I'd question the thoroughness of a shop that replaces suspension bushings on one side only. The correct thing to do is replace both sides at the same time. I'm also puzzled by the choice of shocks, since Bilstein comfort shocks are the way to go.

In any event, a vibration that starts or gets noticeably worse at 55+ is a classis symptom of balance problems, and your trip to the dealer may take care of the whole thing.

Regards,

I agree, sounds like a tire/wheel problem to me, too. Check tires/wheels for roundness. Seems as something upsets the balancing of the tires/wheels.