Hey folks,
This is my first post here but I've been reading for quite some time.

I have a 2016 C450 that has become a nightmare at my local MB dealership (still under CPO).

Current issue: New issue - didn't happen before or get worse, rather just appeared. Vehicle vibrates pretty hard when accelerating or decelerating. The worst of it is between 2000-2500 RPMs. It happens in all gears. It happens worse if decelerating (downshift and when it hits 2500rpm, it vibrates hard until 2000rpm).
Do any of you have any suggestions? My dealer first said, it's the exhaust (story about that below) - something in the resonator. Then saying it may be an engine mount and it's not covered under CPO (which is contrary to the CPO document that clearly states engine mounts are covered).

I have a bad taste in my mouth when it comes to this dealership and here's why:
I brought the vehicle there with symptoms of wastegate chatter. I could hear a rattle sound on coasting, accelerating and decelerating. MB first told me it was my exhaust - I'm easy, so I buy a new cat back (they confirmed it was after the cat) straight piped stainless exhaust. Problem still there after spending $3,000. I bring it back in, they now say it's "normal". Problem gets worse so this is clearly not "normal" and I bring it back in and told them the tech needs to drive with me. After he drove with me, they agreed to replace both turbos. This was a solid 6 months of going back and forth and leaving my car for weeks. I told them on the very first visit that this seems to be a wastegate issue where they said "We checked, and they're both fine". Low and behold, after both turbos are replaced, the problem is now gone and they confirmed both wastegates failed.

I'm beyond frustrated that I bring the vehicle in saying it's vibrating on deceleration and they told me it's my exhaust (I put stock back on for a road trip last fall). I told them to put their money where their mouth is and that if they truly believe the vibration to be from my resonator in the exhaust, I will bring them a non-resonated straight piped exhaust to install. If that resolves the issue, I will gladly pay the install cost. If it doesn't fix, I will not pay. Seems pretty fair to me but they have yet to accept.
They now say they're going to drive the car today (5 days after I dropped it off). They say it may be an engine mount but not covered under CPO which really doesn't make sense because I have the CPO coverage right in front of me and clearly states that engine mounts are covered.
Another good story about this specific dealer is that I brought the car in with a rim that kept leaking air even after I put brand new rubber on it (kind of tells you it's the rim and not the rubber) but they told me twice that it's fine. I bring it back a 3rd time and get upset saying they need to fix it right once and for all and they found a huge crack in the rim (I bought a new rim no issue, I just want things actually fixed, not nickle and dime them). So the first two times they admitted they didn't even dismount the tire and time, just jacked up the car and sprayed with soapy water. Come on.... I'm not even a mechanic and I did that in my driveway, I didn't expect a MB dealer to do the same thing. They didn't even look close enough to see a crack in the rim which was over 3 inches long and very apparent if you actually looked at it. Ok, my rant about them is over.

Does anyone have any clue what this could be? I was thinking possibly flywheel, engine or transmission mount, or exhaust mount? When driving, if I downshift and let the RPMs hit over 2k and let off the gas and let them drop, as soon as it hits 2500 (or very close to it), the car vibrates where I can feel it, hear it and so can any passenger in the vehicle, until it drops under 2k and then it subsides. It does this in every gear and also on acceleration just not as bad as deceleration. I'm beyond frustrated that I can't just drop my vehicle off to my local MB dealer and trust their word. Since they are now looking to me for suggestions as to what it could be (I work in technology, I'm not a mechanic), I need some help....

If you need any additional information, feel free to ask - I'm just looking for any input that may help.

 

Time to head to another dealer. You could also call MB corporate to elevate resolution.

 

I think so, just sucks because the next nearest to me is 50+ miles away from me and owned by the same folks as my local shop.

 

I'm gonna say motor mount

 

My initial thought when you mentioned vibration was a possible driveshaft issue, but after reading your description I'm not sure that's it is if you feel it only at certain rpm and when the car is downshifting, etc. Ive heard of others having to replace a motor mount due to vibration.
If you feel a harmonic vibration or humming noise around 60mph it could be an out of balance driveshaft

 

Yeah, it’s only at a specific RPM range. If I downshift and it hits say 3500 RPM, it will not vibrate until it hits 2500.
I find speed is not a factor, I’m quite technical so my brain started testing certain factors. If I’m doing 75mph in 4th gear at 4500rpm (example only) it does not vibrate. If I’m doing 75mph in 5th gear at 2500rpm, it will vibrate. This is the same for all gears/speeds.

MB is now replacing the motor mounts so I’ll give an update after that’s complete.

 

@C450NY Mount replacement completed?

 

No word yet, been 2 weeks since I dropped it off. I reached out to my SA at the dealership with no response. I’m going to ask again tomorrow of the status.
I have a feeling they’ll say it’s done but not test the vibration. I’m going to ask to drive the vehicle before I take it back home as I can replicate it in the parking lot.
I’ll keep this updated, just waiting forever for what I thought was a quick job.

 

Well I picked up the car this evening and the vibration is gone.
What they did: 1) true all 4 rims (remove bends, back to round) 2) replace all motor mounts 3) updated software (i think this was unrelated)

I’m so happy to be able to say the issue is gone and both the shop foreman and I agree it was more than likely the motor mounts as one had some decent play in it after removal.

Whatever ECU update they did seems to really have helped how the car drives. Shifting points more consistent, smoother start from stop, etc.

Huge thank you to everyone on this thread that jumped in. This was my first time needing some help and it’s awesome to see a community that is willing to jump right in to help, so thank you.