JayDW204

Hi i have a 2011 C250 V6 2.5L engine is M272 the car has always been very well maintained always at the dealer never seen winter and oil change always done at 4-5k KM and recently it started knocking only when its on idle.

example: you can barely hear the knocking on idle but if i rev it to 2k the knocking will go away and when i let go off the pedal it knock faster and louder.

Also : All the pulleys have been checked and belt is new, tensioner is moving a litle bit on idle but i dont think it can make that type of knocking.

Here is a clip of what it sounds like on idle unfortunatly i could not record a sound when deceleration for now but at the end you can hear kinda how rough it stops when i turn it off.

https://youtube.com/shorts/L_GCBz1YVCk?feature=share

MJ50

moved to W204 forum...

Odd Piggy

How many miles (km) are on the car?
Since it’s idle for half the year, it’s probably low mileage.
It sounds a bit like rod knock, but that engine has low reciprocating mass and very short stroke. That and low mileage would probably indicate looking for another cause first.
Since it has been dealer maintained, have they offered an opinion?

JayDW204

It has 151K KM or 93K Miles and the dealer suggested to change a couple parts like alternator or the whole pulley kit since there is nothing showing on the scan but i rather find out exactly what it is before replacing everything in there.

Odd Piggy

Since the dealer tech has been under the hood and offered advice that it’s one of the rotating parts on the front of the engine, let’s go from there.

You did mention that the tensioner pulley moves a bit when the engine is idling. That could be from a serpentine belt that is beginning to separate. It’s hard to see without examining it off the car. If that’s happening, the belt needs replacement.

A second possibility is a cracked plastic pulley. On my since retired E320, there was a clicking/tapping noise under the hood that I could not locate until I noticed one of the plastic idlers had a crack in it while changing the water pump. I suppose it would have eventually failed.

Another thought is a bad bearing in one of the idlers or the tensioner. Bad bearings usually make a roaring noise, but anything is possible.

I would think the alternator is a remote possibility. It can be checked with the belt off.

The tech probably considered replacing everything rotating to be more efficient (for them, not you) to get the offending part out at $200/hr, rather than taking the time to locate exactly what causes the knock. The idlers are not costly, the tensioner is medium priced, but the alternator is expensive. The power steering, water, and air injection pumps are also parts of that ballet. Changing parts until the noise is gone could get very expensive.

Are you equipped to do any of the detective work yourself?

JayDW204

Ok yes makes a lot of sense for the pulleys because i think it is a kind of rotating click or tick or something like that and i just realised that the noise only appeared after work had been done on the belt and tensioner.

Will check that out and give a update. Thanks

JayDW204

Found out what the problem was, it was caused by the tensioner installed a month prior turned out it was faulty even tho it was a brand new one.

the tensioner spring were bad so it ended up jumping while the belt turning on idle.

Had the tensioner, belt and pulley kit replaced and the car makes no noise. Hope this helps anyone with the same issue

Odd Piggy

Or maybe the tensioner was not set correctly when it was first installed??? There is a necessary procedure to release it after installation.
Thanks for the update.