Greetings ladies and gentlemen,

I used to be way more active on this forum but life got so busy and I had less and less time to stay active.

My car (2012 C300 4MATIC Sport with 84k miles/135k km) has had the following problem for the past 2 years. It used to be much more mild but now it's more serious/severe.

With the car in Drive or Reverse and the brakes applied and the car NOT moving, for example while stuck in traffic or as I am backing into my parking position, I feel a numbness/vibration through the steering wheel, brake pedal, and doors. Definitely nothing felt through the shifter. As I release the brakes and the car starts rolling forward or backwards, the numbness/vibration disappears. Back in Dec 2023 when I first started having this problem, my mechanic said it's the engine mounts since 4MATIC cars are known to eat through engine mounts faster than RWD/FWD cars. We replaced the mounts at 68k miles/110k km with brand new Meyle mounts since OEM mounts were not available but the problem remained. We then changed the transmission mount with an OEM one and the problem remained. I was then told to let the numbness/vibration grow stronger so we can more easily diagnose the problem.

A year later, during an inspection, we found that some axle boots in the front were ripped and needed replacement. We replaced the boots but it turned out that the axles were damaged already because of the grease spewing out so I replaced both axles with Frey axles since OEM axles were not available. Again, this did not affect the numbness/vibration which continued to increase bit by bit as time passed by.

I kept driving the car as is until last month when the numbness/vibration became noticeably stronger. I had the car inspected by three separate mechanics at three different shops and they all diagnosed the problem as engine mounts. This time, I made sure I source OEM Corteco mounts from FCP Euro and we replaced the mounts and the problem is still there. We tried a new OEM engine mount again and same thing.

With the car on the lift and numbness/vibration active, we tried jacking the engine and transmission from all sides and nothing we did could get rid of the problem. We inspected the Frey axles and they are 100% fine.

I am becoming increasingly frustrated with this problem so I thought I'd write here and see if you guys can thankfully help.

If you want to have a look at the old and new mounts or any other repair work mentioned, let me know and I can post photos.

The car obviously has no error codes whatsoever and drives flawlessly through all RPM ranges and gears. The problem only occurs at a standstill and I can make it worse by having my foot on the brake and increasing the revs a bit in D or R.

Thank you for taking the time to read this long post and I look forward to hearing from you.

 

Have you inspected the harmonic balancer/crankshaft pulley? It has a rubber surround that can crack and it would cause vibrations.

While diagnosing the issue is it coming from the engine? I would assume as someone is sitting in the car, engine running, car in drive foot on brakes not moving someone else can feel the engine and see if its vibrating abnormally?

May be a fuel related issue. Keep an eye on the rpms when this happens. A Mercedes specific obd scanner will come in handy.

Once in awhile i'll feel a slight vibration at stop lights, like the engine isnt running as good as it should be. Nothing crazy but something I notice. I will buy a big bottle of Techron and use it in the gas tank. I would give it a try if I were you. May solve the issue or make it a little better. It wont hurt. 1oz Techron per gallon of gas. I will measure it out, I have an empty bottle of techron I will put 10oz in then at the gas station put in 10 gallons of gas. I do this twice in a row.

 

Torque converter, maybe? Problem occurs when all drive train components are stopped, engine is running in gear. That’s the component absorbing the unused engine torque when the car is in gear and not moving. Engine and transmission mounts are the first thought, but those have been done.

 

No I have and I don't have any related codes but I will definitely take a look at that.

Yes I can feel it's coming from the engine because any slight steering input while the numbness/vibration is present will cause it to modulate meaning it will change a bit in frequency or amplitude. If I touch the engine, I can definitely feel some numbness/vibration vibration.

The RPMs are normal and the fuel trims and ignition timings are normal/symmetric as confirmed by the MB OBD scanner I have. I couldn't notice any anomaly there and obviously no codes at all. I was even suspecting a bad coil but no error codes ever. Since I have a brand new OEM coil, I was thinking of swapping it around to see if this could fix the problem.

I add Techron in the gas once a year. I dump the bottle and follow the instructions written on it. I could try the dosage you suggested.

 

I took it to a transmission shop and they tested it at standstill and took it for a ride and they concluded that it's not the torque converter because they said if it were, the problem would remain, at least for a bit, as the car starts moving. They said if the problem is only apparent while stationary, it's definitely not the torque converter.

 

What tire pressure do you run? If not the engine or the transmission maybe something in the steering rack is loose or worn. High tire pressure would make this more evident.

 

OP states that the issue is when the car is stationary.

 

So it happens in drive or reverse, and it seems rotating components have been validated except the harmonic balancer (I agree that needs checking)
Has anyone examined the exhaust mounts? If worn or loose, it is possible they could contribute to a vibration. Does the vibration go away or lessen if you gently accelerate and raise RPM in drive with the foot brake applied thus holding the car stationary?

 

I run the tire pressure on the door jamb. 36 in the front and 41 in the rear.

 

The harmonic balancer will definitely be the next thing I check. Thank you.

We inspected the exhaust mounts when we replaced the engine mounts since the entire exhaust was removed. Everything checked out.

If I gently raise the RPMs in drive or reverse with the foot brake applied and the car stationary, the vibration becomes worse in direct correlation with the RPMs and you can feel it more everywhere (steering wheel, brake pedal, doors), especially if the car is on an incline (forward or backwards). The vibration completely goes away when the car starts rolling as the brakes are released.

 

So I took my car to a new Mercedes-Benz master technician and he ran a lot of diagnostic checks for the engine and transmission and he said everything checks out.

We then put the car on a 2-post lift with me inside it and I was able to replicate the vibration in reverse and drive while he worked underneath. As soon as he started tightening and loosening the exhaust bolts, I could feel the vibration easing and increasing. The exhaust bolts were definitely a key variable in this equation. He asked me to shut down the car and to stay in it and he worked on re-aligning the exhaust for like 30 mins. He then said the problem should be fixed.

When we took the car for a test drive, the problem wasn't gone and it actually became worse on a couple of occasions. This allowed us to verify that whatever he did made the problem worse and to confirm that the exhaust is indeed the culprit. He asked me to bring the car back when I can let it spend the night in the garage so he can work on it first thing in the morning when all the components are cold. He said re-aligning the exhaust when it's hot will not yield the precise alignment needed.

I will report back when I can leave the car there overnight for him. Thanks.

 

Interesting. I agree with @Odd Piggy that it could be the TC, but it seems now to be the exhaust, or at least the exhaust is a contributor.

If the problem is something like the harmonic balancer or TC, you may not get any codes. And definitely not if it is a misaligned exhaust system.

You did not say if the vibration occurred when in Neutral, but only in D or R. That means the moving parts are the engine and TC. Remember, the TC is essentially a hydraulic coupler that uses the viscosity of the transmission fluid to "couple" the movement of the input and output shafts. I'm not a transmission expert, but if all the alignment stuff doesn't work, that would be my next place to look.

I am assuming that when your transmission was serviced that they used the correct fluid. Since you have the 7G+ transmission, they should have used the blue fluid.

 

Kinda simplistic, but interesting as well...

 

I've seen situations where the exhaust is the culprit before, which is why I mentioned checking it several posts back. The situation is that stationary in reverse or drive components like the exhaust take up the torque between engine, transmission and body that is relieved when the car actually moves. V6 GM Calibra autos (EU vehicle) were highly prone to this. It looks like the exhaust and mounting system are indeed the root cause here.

 

Interesting. From my understanding the exhaust is attached using rubber mounts that should remove all vibrations. Im assuming your exhaust is similar to mine, dual exhaust pipes connected together in the middle at the resonator. The two cat pipes have brackets on them that mount to the TC housing.

I replaced the two clamps leading to the resonator and the clamp attaching the drivers muffler because they were rusted and leaking.

The flange on my passenger side has disintegrated and needs to be replaced. I have seen this same condition on used resonators when searching on ebay for a replacement. Unfortunately MB will only sell the entire section with resonator and passenger muffler for around $950, they dont have just a replacement flange to slide or weld on.

I dont have any abnormal vibrations with my failing exhaust. I've even been under there disconnecting various pieces. I dont see much adjustment being possible.

 

In my early days of modding cars, I replaced the "dog bone" mount on a VW or Audi with one that had stiffer bushings and the vibration would scramble your brains. I also installed a 3" exhaust which would also rattle when at idle because of the close tolerance between the exhaust pipes and the undercarriage. So, checking the exhaust system first is the way to go.

One of the symptoms of a failing TC is the increased heat. It's definitely worth observing the transmission fluid temperature using a scan tool to see what it is. The transmission temperature for my car with the 7G+ transmission rarely, if ever, exceeds 180°F once warmed up.

 

The master technician actuated the TC using the diagnostic tool and he checked all the parameters and he said it is working 100% perfectly.

I service the transmission every 60,000 km and I make sure it's the blue fluid (FE) and the 7G+ enhanced filter.

 

That is correct. Same setup on my car but rust-free since the weather where I live is mild and no salt is used.
Thank you for posting the photos. They are helpful in visualizing things without having access to the car's undercarriage.

 

I don't think I've seen it that high but I will connect my diagnostic tool and monitor it to see at what temperature it maxes out. Thanks.