For the last few days, I've been hearing a distinct single KNOCK sound coming from the driver footwell area when accelerating from a stop. It would usually show up at 15-30 mph and seemed to appear only when accelerating in a straight line. I brought the car in for service today and watched the service tech take it out for a test spin, then bring it back into the shop (the dealer has a glass wall in the waiting area that overlooks the shop). He parked the car, opened the hood, then gunned motor while another tech observed. Yikes, the driver's side of the engine rose up what seemed like 8-10 inches out of the engine compartment (that may be exaggerated - hard for me to judge since it was 75 or so feet away and I was viewing from the second floor down)! Regardless, I'm thinking broken motor mount for sure.

While waiting for the repair, I visited with the accessories manager, who's always treated me great. I mentioned the symptoms, and while he had nothing to do with the repair, he later checked with the service techs and reported that while the motor mount was fine, its bolts had somehow worked loose and dropped out!

I mentioned that scenario to the service manager handling the situation (a backup to my usual svc mgr who was out sick) and he would only say that they had identified the problem and the car would be ready shortly.

When I received my invoice (covered under warranty, of course), there was no mention of motor mounts or motor mount bolts. Instead, it says "Left front sway bar nut is loose, need to R&R LF sway bar nut and apply Loctite to thread."

Anyone ever had a similar experience? Seeing the driver's side twist up 8-10 inches out of the engine compartment when the service tech gunned it sure sounds like a motor mount problem to me and nothing to do with loose sway bar nuts.

Regardless, whatever was done, the symptoms have disappeared. On the way home I started off with part throttle launches, increasing to full bore, with no trace of the "knock." Knock on wood things stay that way - this week is the one year anniversary of the beast and apart from this isolated incident, it's been trouble-free.

 

Seems pretty obvious to me that the mount was the problem. Broken mounts were not to uncommon for W210 55's, but I've never heard of a W211 having that problem or the bolts falling out.

 

I have heard of the locktite on the swaybar mount thing -- but 8 to 10 inches?!!! Nah. My completely uneducated guess? Motor mount. What else is going to allow the engine to move that much vertically?

 

Agreed. Given the angle and distance, it was hard from my vantage point to accurately judge how much the motor actually rose out of the engine compartment. The 8-10" is purely a guess.

Anyone with like info, please post and I'll update my experiences accordingly.

 

wow that is a really interesting problem. does sound like a motor mount to me, but since they put a swaybar bolt?? i dunno whats goin on. good to know the problem has been fixed and hopefully it was just that the loctite had worn off the bolt over time and that it wasnt the start of some serious problems.

 

Woa ... I've been touting "check/change your motor mounts" big time since this summer when after changing my motor, and using new E55 mounts, discovered how more solid the car was, and how the wheel hop finally stopped and how... etc... But what a knuckle-head I am that I never connected the dots to realize that that sound/feeling under my feet you are describing was also cus'a the motor mounts ... in retrospect, what I had suspected is now confirmed, that I must have had broken mounts for over 50K miles, as I had that sound for a long L O N G time. Whoa...

Regarding what showed up on the Invoice, I'd press them as to the real reason why. No way a sway bar affects the motor, when revving it or not, while that motion you describe is exactly how you diagnose a broken (or in this case, unbolted) mount.

 

Sounds like your dealer is scamming MB for whatever reason I don't know. Maybe for more reimbusement? Just a thought.

Problem no doubt is a motor mount problem IMO.

Have you had any work done on the motor whatsoever?

I had the dealer change out a bad sensor when my car was brand new and one year later had one of the fan belt pulleys get loose and almost fall off. I can't imagine that coming from AMG like that. Must have been an incorrectly torqued bolt during that repair. Bet you that's what happened.

 

As a matter of fact, after the repair, the service advisor asked if I'd had any work done to the motor (I haven't - yet ). Definitely agree it was motor mount related, but why they'd put it down as a sway bar nut problem baffles me.

Likely scenario is under-torqued motor mount bolts, but I'm surprised the problem never showed up before, especially at the track. Remember, while the engines are built at Affalterbach, they're installed in the chassis at Sindelfingen, along with the non-AMG M-B's.

 

...maybe yours/now... :v