I was driving in the mountains yesterday and noticed a strange rattle or metal grinding noise from the front axle or brakes. It happens only when the car is moving and I lifted my foot from gas pedal, or when I brake a little bit to slow down then release the brake pedal. Once I stepped on the gas pedal the noise disappeared. I suspect there’s something wrong with the brakes but haven’t taken off the wheels to check that yet. I saw another post online before saying it could also be the Predictive Brake Priming issue but I’m not sure if my car has that feature( is it in the pre-safe package?).

Another concern is if it has anything to do with differentials. Although the noise happens while the car moves straight and not turning. I just changed front and rear differential fluid recently but I didn’t use MB ORM parts, which is 75w/85. Instead I used Liquid Molly synthetic hypoid 75w/90 ordered from FCP Euro. Their website says it fits my car. Would the weight of the oil make the difference? The OEM costs $90/liter from dealerships which is crazy. I don’t want to spend that much money before I exclude the brake problem first. Are their other options for the fluid? It’s difficult to find synthetic 75w/85 hypoid oil other than the dealership.

 

Sounds like a wheel bearing, but I'm guessing, maybe post year and model to help?

 

@pierrejoliat 2013 E350 4matic. No any issues before. Replaced the differential fluid recently but didn’t drive until yesterday. Drove to the national park for an hour on the highway from home without noticing any problem. But when in the park going downhill and applying brakes I started noticing the noise.

if it’s wheel bearing does it have anything to do with differential fluid change? I’m pretty sure I added correct volumes. Only problem is not the OEM fluid.

 

No, I wouldn't think so, did you drain and fill from the drain hole? Aftermarket approved fluids are fine, can you jack up the car and spin the wheel you believe the noise is coming from?

 

​@pierrejoliat yes I did the front differential from the drain hole without removing front axles. Approximately 550 ml for the front and 1.2 liter in the rear. I will jack up the front and rear to check wheel spins and maybe the brake guide pins too.

 

Good call,

 

@​pierrejoliat so I took off the front wheels this afternoon, cleaned and lubricated guide pins and other parts of the brake assembly then test drove it again. The noise is still there and sometimes loud. I asked my wife to drive it and listened outside. I figured out that the grinding noise is from both brake pads rubbering discs. Now I can exclude the axle and differential fluid problem.

I noticed that the front discs are not evenly worn, the outside are worn more than the center. Just wonder if it’s because the master cylinder didn’t retract well after braking, or there is something wrong with caliper? I plan to do a brake fluid flush first and maybe change front pads too, although they are 3/4 left. Anything else could be wrong?

 

Wow! Definitely the pads are not contacting the rotors properly. When were the pads replaced? Are they OE or aftermarket? You have quite a lip on the edge of the rotor; you probably need to replace the rotors and pads. Not sure why it would suddenly show up, but those rotors look like this has been going on for a while.

 

Pads and rotors are both MB parts. Pads replaced about 2 years, and drove for 20k miles. It hasn’t been driven recently and had some rust on it. It had been driven in snow/salt condition last year. No problem until last week weekend

 

Cleaning and lubricating guide pins alone is not sufficient if debris or dirt or rust on the piston of the caliper already happened.
However, unless brake pad is proper flat and disc rotor is also proper flat ( yours is not ), the automatic or self return of the caliper piston distance is not enough.

Caliper piston automatic return / retract or lift up is purely by the flat lip seal design on the caliper piston and its so so very small lift up distance.
The seal relies on deflection only as a way to pull piston back in and your brake pad away from rotor .....you see.
https://www.freeasestudyguides.com/b...ston-seal.html




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I will give you an example how I "tuned" my OE Brembo 4 pistons ( 2 + 2 ) to have zero touch brake pad to disc rotor.
If your caliper is a single piston only, you can never achieve a true zero touch because the actual compression is single sided, the side without piston is just a follower, example my rear caliper.

Read here : https://mbworld.org/forums/e-class-w...y-achieve.html

When you done reading my post, you by now should understand when and if the disc rotor is not pure equal flat for its entire surface,
it is a challenge for the caliper to do its proper job.

Also rust build up at wheel hub to disc rotor is a BIG ISSUE. A mere 0.3mm un-equal mating surface height due to steel "expansion" by rust will make the disc rotor
becomes also NOT FLAT for the brake pad.

If you have winter salt, and in the first place the wheel hub has never been protected using anti seize grease for corrossion protection, it will rust.

Looking at your disc rotor, you need a new one and a new brake pad too and this time make sure the wheel hub get proper corrosion protection.


The very first time I removed my disc rotor, I need a tracker, this is simply because of the rust.




This rust is like power glue Not cleaned yet.


.CLEANED



CLEANED, but not yet get the anti-seize coating.







After that I apply a good high temperature anti seize . Don't let the grease contaminate the wheel hub 5 female threads for the wheel lug/bolts.

The grease coating is very very mild but even.


The next time I remove my disc rotor, no need anything, no hammer nothing....simply pull it out with my hand.

The lifetime of a typical front brake disc rotor is about 1.6 to 1.8mm only. Yes very very "thin" life. Maximum 3 brake pads,



This is the height deviation the rust caused. I use dial gauge to measure it.


AFTER RUST CLEANING






Use a boroscope ( if yours is a 4 piston ) and see if your piston caliper dust covers are torn or not ?


DUST BOOT







Hope you can sort the brake thingy soon....

 

8 months later and a few thousand kilometers later.
This is how the anti-seize coating looks like




.



.




========================


I replaced the disc rotor at this point in time and new brake pad ( must ) due to overheating at the race track back in 2020 and it was no more smooth since then.
Even my brake pad was damaged








.




.




.
.New disc rotor vs old one




Machining line on new disc rotor





.

 

@S-Prihadi thank you so much for the detailed information. That’s very helpful! Mine has a single front piston. The center area are pretty rough due to slat corrosion. I will get a new set of rotors and pads and clean the other parts well before installing. I believe the problem will be solved once the new parts are installed.

 

Okey dokey, glad to help

 

If you get your parts from FCP Euro, all future purchases will be reimbursed. So, even though FCP may not be the cheapest initially, they are in the long run for parts that are wear items.

 

it will take at least a week to get them shipped to me. I’m going to call local dealers today to see how expensive are the parts. If it’s just a $100 different I would just get them and fix the problem this weekend. But most likely the price will be almost double.

 

FCP also offers OE brands, meaning they are exactly the MB Genuine parts without the 3-point star decal. FCP does offer expedited shipping. And then there is always Amazon if you go with a name brand. The Mercedes dealer (as well as your local parts store) is likely going to charge you A LOT more.

If you are in DC (as your name implies), FCP is pretty quick to deliver to Maryland, always ahead of schedule. But, if you want to do it this weekend, you will pay through the nose. Personally, I'd wait a week if I had to and just drive carefully until then. Doubtful you are going to hurt anything.

 

Yes, those rotors are toast, wow, as Jetta said the contact is minimal pad to rotor, I would suggest the Zimmerman solid rotors and Akebono ceramic pads from FCPEuro. looks like you had semi-metallic pads installed and they filled up the holes somewhat and created problems, that happened to mine on my annual 2,000 mile trip to Arizona, I hit them hard a few times and they got worse the farther I got, by Tucson they were warped and grooved badly.

 

It is, patience will save you 60% over dealer, its a week, up to you

 

Yes I’m in DC. I wish I was in CT so I can go there to pick up the order myself …

 

It's going to be too damned hot to work on your car this weekend, anyway!