C300NYC

2015 C300 Sedan, 40K miles.
Local dealership including service manager, shop foreman, etc. have so far failed to properly diagnose and repair this issue. Intermittent chirping sound (see video) from front end of the vehicle. Three separate repair attempts, but the issue re-occurs every time, typically after about 15-20 minutes of driving.
Things that have already been tried:

(verbatim quotes from Mercedes dealer invoices given all the amazingly snarktastic comments below):
Repair 1:
CHECK & REPLACE ENGINE BELT TENSIONER---REPLACE ENGINE BELT-----REPLACE GUIDE PULLEY---DONE

Repair 2:
1. BP OK 5HR ST CF 5/31/2024////CHECK & VERIFY CONCERN, #1-CHECK VEHICLE & FOUND WATER PUMP TO
BE NOISEY-#2-DRAIN COOLANT-#3-R&R AIR CLEANER & HEAT SHIELD-#4-R&R INTERCOOLER-#5-R&R-
ELECTRIC FAN & RADIATOR-#6-CHECK & REPLACE WATER PUMP, & REPLACE CHAIN -TENSIONER.
---


Every time the car is supposedly "fixed" the issue simply re-occurs. We are $4,100 into this marathon and at some point it won't make any financial sense to keep pouring money into this 2015 C300, low mileage and all.

Any insights or suggestions?

CaliBenzDriver

this is a weird engine you got with a timing belt tensioner and a fan belt as well. So many options available...


let me hear all that now.....

I do hear minor chirping.
40kMi low mileage
new serp. belt + tensioner.

Do you know the first thing that fails ???
the dried plastic pulley idler bearings.

but you are wise not to blow money on untested conditions.

> Quickly test all pulley free play and rotation: 10mn

> Plastic PCV valve is also famous for chirping.


Perhaps your $4k failed repair is an excellent opportunity to go find a worthy specialist where techs can fix cars.

JettaRed

iTrader: (1)

Well, I can't hear anything in the videos you attached.

The dealer actually told you they replaced the timing belt and pulley? You do know there is no such thing on your car, don't you? Your fan is electric. Also, you don't have a timing belt. You have timing chains, and though they have tensioners, they don't squeak when needing replacement. A water pump bearing may squeak, but usually they leak when they go bad.

You're right! You don't want to keep giving these people your money. I'm afraid this dealership has scammed you. Maybe post your invoices and pictures of the front of your engine so we can be sure you got things right. If you have portrayed this accurately, then maybe you need to see if NY state has a consumer protection commission in the AG's office.

JettaRed

iTrader: (1)

Cali, I have a 2014 C350 with 128,000 miles on it and mine purrs. I want to see pictures of the invoices to know what they actually replaced.

Hmmm, my bad. He has one of those 4 cylinder C300s. Maybe it does use a timing belt and a belt-driven motor fan. Need to do some more research.

JettaRed

iTrader: (1)

Nope. I was right. His engine (M274.920) uses a timing chain and electric fan.

CaliBenzDriver

I think they switched back from rubber Tbelts to chains because Tbelts would not tolerate the extreme heat of limited oiling under 3500.Rpm.

The dealer has been focused around the belt area.

-- I don't think oil pumps can fail so quickly.
Perhaps something is misaligned from a washer being left or missing on idler pulleys or tensioner.

-- Perhaps wrong belt length is used and tensioner over/under extended.


A video with engine hood opened would save time. Watch out for moving parts grabbing loose garments...

JettaRed

iTrader: (1)

There's nothing in the WIS or EPC about timing belts for that engine.

Chevota

Sounds like an idler bearing for the belt to me, so I don't blame their opinions. They replaced all of them, correct?
Next would be the alternator, which I doubt, because it doesn't sound right, but it's from a video so I'm sure it's not right. AC pump, maybe, but more doubtful. Any other stuff in there feedin off the belt?
What I do with mystery noises like that is douche the parts with oil one by one. WD-40 will be fine. With the pulleys you may need to hit the back sides of them too, and no guarantees anyways if they're truly sealed, but worth a shot. If all were replaced then I'd go straight for the Alternator and douche both ends at each bearing.

Optionally I'd take the belt off and spin the pulleys by hand. If one is F'd up enough to make noise you'll probably feel a difference by hand. Optionally spin it with a tool at speed, that'll make it cry.
Or, let it do it's thing until it fails and becomes obvious.

C300NYC

Here it is. Chirping sound clearly audible towards end of video

CaliBenzDriver

I am going to bet a plastic idler pulley has gone dry. Once hot the light grease evaporates to make bearings dry and noisy.

You can lock the belt tensioner and test the idlers for looseness or replace kit at once with :
serpentine belt
tensioner
idler pulleys

Kit under $100