Purchased a C43 sedan, 2017 at 178000km. The vehicle seems really good and healthy compared to the kilometers, however I'm a bit worried about one particular noise from the engine.

After cold start when the rpm drops to the normal idle level, I can hear a knocking sound that quite distinctly comes from the left bank of the engine (viewed from the front of the car). It's very noticeable when listening with a screwdriver and connecting it to the left valve cover. It's so powerful that it almost hurts my ear when I listen through the screwdriver and I can also feel a sharp impulse in my fingers. What bothers me the most is that the right bank seems pretty much silent in this regard (also no feedback to the fingers there). Of course both of the banks have that typical healthy timing chain whirring sound, but on the left bank there is also the knocking that is clearly a separate noise.

I can't hear any timing chain slap when cold starting the engine. To me that knocking rather sounds like a valve thing. What do you think?

From the cabin I can hear something that I think is the same noise, but becomes more muted obviously. So the idle sounds like sort of deep and soft knocking from inside the car too. I can't hear the noise when I rev the engine even a tiny bit. Just to be clear, AC is turned off so it's not that. Also this is not a purge valve, I know how it sounds like and it's located in a totally different side of the engine.

The noise fades a lot when the engine warms up but is not completely silent.

Better when listened with a decent headphones:
https://www.dropbox.com/scl/fi/gyn7n...=wvplk8uq&dl=0

Also found these from Youtube, sounds pretty much or exactly like these two:

 

Valve or ....piston.

 

Those are C 300 with the M274 that sounded like the wrist pin if I recalled correctly, a non issue on the M276. Not sure why but I couldn't hear what you are talking about in the dropbox link. Could you also upload to youtube?

 

Sounds pretty much exactly like those c300. I don't think youtube makes any difference. I had this issue with my friend too, he didn't hear it and it seems to depend on the device you're listening on, but his headphones were absolute crap. I can hear it on my phone, monitor speakers and headphones though..

Here I tried to isolate the noise with a video editing software and now it's pretty obvious at least if you know what to listen. Now the sound breaks and sounds kind of high pitched at times because the phone mic can't handle it (my phone was basically touching the screwdriver head).

https://www.dropbox.com/scl/fi/ol1qi...=dlakphaf&dl=0

 

Sorry about that, maybe I have bad ears.
See if this is similar to your sound : https://mbworld.org/forums/e-class-w...76-engine.html

 

If you need further assistance, forum member CaliBenzDriver is the M276 expert and he will be more than happy to help you : )

 

This indeed sounds very similar. I mean the deeper clap on the background, not the injector tick that is normal.

 

I see. Hopefully you find your answer there : )

 

Forum member CaliBenzDriver calls it a tapping sound.

 

Yeah it seems like the whole discussion turned into oil viscosity but the tap tap tap noise never got explained further.

Seems like there is the water pump attached to the left cylinder bank side (mostly) so I might want to suspect that too. But it's a bit more tricky to listen to..

 

He did mention it could be belt related apparently

 

The belt area seems impossible to listen with the screwdriver method, it's so packed. But I listened from the rear end of the valve cover and the noise is loud there, perhaps even louder than from the chain side. Something tells me it's not the pulleys.

New video:
https://www.dropbox.com/scl/fi/g19u8...=ul9tutzh&dl=0

 

I see, let me see if I can hear this one.

 

OP, I finally hear what you are talking about. I will need to compare the sound next time I drive my vehicle in a week. In the meantime, hopefully someone chimes in soon.

 

I am sorry as I am away for a while....
At 180k-Km this sounds like a genuine concern

Thin oil film is producing racket from engine head with pressure loss caused by increased gaps.

Overall the tapping sounds like pistons
to be confirmed with compression test and borescope


These tests will further tell you if :
-- this is survivable with better oiling
-- or terminal... (check oil dipstick for glitter)


This engine has passed its healthy prime. Take it easy to try extend its useful life.

🤞

 

Comparing to piston slap videos of a Subaru etc. I can confirm it sounds exactly like that (albeit not that loud yet), also the fact that it kind of fades away after warming up. Still hearing it from the cockpit though, cold or warm engine.

I used to have similar suspects with a high mileage Honda some years ago. After weeks of forum searching, it ended up being just a "cosmetic issue" that didn't need further attention or so they say. The pistons are so flat nowadays, the slap is incredibly common. Honestly I've heard a fraction of that noise in every Honda after that (before moving to Benz), on cold mornings. Still they keep running seemingly forever.

Now the c43 is a performance vehicle so things are obviously a lot more critical, but do you think this could be partly "normal" due to the mileage of the engine? Also I did the oil change about 1 week ago and it didn't affect the noise at all. After all, the noise is very faint and barely noticeable without close listening, so I think it's only a beginning.

Edit: Might add that it's quite common here in EU to drive these cars over 250k km or even more. I've seen a couple of these for sale at that mileage with the original engine parts. The only critical issues during that journey were said to be related to 4matic and leaking valve cover sealant/gasket. No timing chain rattle, nothing. But I don't think most people would notice a faint piston slap so who knows.

 

The M276 is a super reliable engine in general so I am not surprised about the mileage examples you mentioned. Honestly as mentioned if you don't have shavings in the oil which I think you already confirmed, it might not be a big deal.

 

Since this noise is louder at the rear (firewall) end of the cylinder head, what's the possibility this is a bad (worn) high pressure fuel pump piston or cam lobe?

 

Well not exactly louder but maybe a bit easier to isolate from everything else like timing chain at the front.
As far as I know, fuel pump noises would not match the frequency of this one, but I don't have much experience on that area.

 

Just noticed that the belt tensioner wiggles quite a lot when the engine is on idle. Is that normal in these? I know they can cause all kind of noises when they wear out, but that would be too good to be true..

 

No, definitely not normal. There should be no wiggle/wobble or any kind of play on your belt tensioner or the idler pulleys.

Here you go:

https://youtu.be/SwEVwZ2GiVU?si=enwMAiLpXRmEp-xE

 

Agree, there should be no play. That is good news if that is the cause to your problem OP! Sounds like what forum member CaliBenzDriver suspected being the belts could be the cause after all.

 

I wouldn't be too optimistic on that though. I've replaced bad pulleys on a Honda before and in the end they didn't make any noise I was hoping to get rid of.

To me this wiggle doesn't look consistent enough to match the tapping noise in the previous videos:
https://www.dropbox.com/scl/fi/ryizu...=o8l121g4&dl=0

Edit: To me that m276 tensioner looks like 100% spring loaded without hydraulic damping? So it will react quickly to any small changes in tension like that, basically doing its job there. To me it's a bit unclear how much wiggle is OK though, because yes in theory there shouldn't be any, but after all these are engines and not perfect.

 

You know... the hard part is to diagnose problems into effective solutions.

The best way is to go-ahead and ELIMINATE KNOWN ISSUES.

The wrong way is to let issues interact to create a snow-ball at which point the root cause is nearly impossible to diagnose.

 

yeah, it should NOT be jumping like that.

Here's mine after replacement.