p.sam118

Hey everyone, writing about an issue I’m facing with my 2015 C300 4Matic, approx. 79,500 miles.

For the past few months now (~2,000 miles worth of driving) I’ve noticed some distinct jerking/knocking when I exert sharp acceleration on the car. This occurs around after around 3,000 to 4,000 rpm and then a visible jerk/knock would occur. After about a couple of times when this happens on a given drive, it feels like the turbo shuts off and then when I floor it, the car gradually accelerates sometimes not reaching past 2,000 rpm.

I took it to a local Benz dealership and they essentially told me no check engine lights were on, couldn’t replicate the problem, and then just sent me home (I ate the like $200 diag fee which sucked).

I decided to change the battery as well to see if that would solve, but still with a new battery this issue still persists. A note as well here is that after the repeated knocking and what I gather as the turbo shutting off, the auto start/stop light is disabled. Usually after waiting some time and after stopping the car and starting again, the drive behaves like normal (until it happens again).

Has anyone faced this issue before or has any ideas on what I can do to solve this issue? I’m taking the car at the end of September for my 80,000 servicing and got a hold of a service advisor at another local Benz dealership with this as a specific issue to examine.

Any information/insights would be much appreciated, thanks so much!

spb147

possible high pressure fuel pump failure?

JettaRed

iTrader: (1)

Your car is going into limp mode when this happens in an attempt to protect the engine from further damage.

For less than $200 you could have gotten your own scan tool and known for sure if there were any codes stored. A CEL does not always light up when there is a problem. Go to Amazon and search for iCarsoft MB V3.0 or CReader Elite Benz. Both are Mercedes specific scan tools for well under $200.

Lastly, it’s good you are looking for another dealer. I would not have accepted their answer. That’s why you need your own scan tool.

JettaRed

iTrader: (1)

By the way, knocking is NEVER a good thing. If the high-pressure fuel pump (HPFP) is failing at high RPMs, you are running lean, which can cause catastrophic engine failure.

p.sam118

Thanks for your reply! I finally got to the bottom of it from a great local Benz mechanic and turns out it was rooted in a crankshaft position sensor malfunction along with a pedal value sensor (accelerator) malfunction. Was a tough pill to swallow but so glad this issue that has been plaguing me for so long has finally been resolved. For good measure, reset the TCU as well after all said and done. Hope to keep this car forever!!

If I ever run into an issue (knock on wood hopefully not lol) I shall buy that reader you mentioned.

CaliBenzDriver

I find it strange unbelievable that accelerator pedal + crank sensor not Limp + MIL.
These two faults are both hard issues that stop the car, not make it run poorly as described.

Get a scanner to help cut through helpless fluff.
🤞

JettaRed

iTrader: (1)

I would not wait until you have another problem. It's good to periodically check for codes because not all trip the MIL/CEL right away. My new favorite scanner is the Launch Creader Elite Benz. While the user interface takes a little getting used to, it does seem to have more functionality than my all-time favorite, iCarsoft MB V3.0. Check Amazon for both. The Foxwell NT510 also seems to be a favorite of some. In any case, spend the money and get one now. It'll be cheaper than towing your car to a shop for diagnosis. Not sure what the mechanic charged, but I'm sure it was more than the cost of the parts. Both the CPS and accelerator pedal are easy DIYs if you know they are the problem.

JettaRed

iTrader: (1)

I do agree that the crank sensor being bad would cause your symptoms. Typically, when they go bad (which was/is quite often with the M113 V8 engine in the older cars), the car dies and won't even start.