I will write a DIY on how to replace the belts/tensioner/and idler pulleys later when I have time as I did not see one on here. I ended up partly following WIS and also read up some of the other forums on M157/M278 for replacing the pulleys.
Here are some pointers I learned when removing and replacing the belt, tensioner, and idler pulleys.

-WIS does not call for removing the fan but clearance is tight, especially in these M278 engines. I ended up removing the fan, overflow radiator hose (which broke when removing. Replacement part # is A1665001091), two small radiator hoses attaching to the top of the engine, and the upper radiator hose (the upper big one on the driver side of the radiator. The fan is only held in my 4 clips and one electrical connector. Once you can maneuver the fan around those hoses, you have so much more room to remove the belt, pulleys, and tensioner.

-WIS calls for removing the charge pipes on driver and passenger side. I only needed to remove the driver side charge pipe as it was blocking 1 of the 4 idler pulleys. DO NOT try to remove the fan then the charge pipe. It should be the other way around. Charge pipe is secured with 2 hose clamps and it wiggles out. Then you have to maneuver it out before you remove the fan. I learned this the hard way.

-No matter how much googling you do, for some reason FCP, ECS tuning, etc only stocks 3 of the 4 idler pulleys. The total parts for this job should be 1 large idler pulley, 3 small idler pulleys, belt, and belt tensioner. FCP only had the large one and 2 small ones. I could not find the other part number for the remaining small one. I went to the dealer and ended up getting it for around $150.

Parts you need for this job:
-Serpentine belt *FCPEuro (A0029936796)
-Large idler pulley *FCPEuro(6112340293)
-small idler pulley *FCPEuro (A2782020519)
-small idler pulley *FCPEuro (A2782020619)
-small idler pulley *I got this from dealer (A2782020219)
-belt tensioner *FCPEuro (A2782000570).

-Coming from a M273 GL550/E550 engine, this M278 engine belt tensioner really stumped me as there is usually a separate hex bolt that is part of the belt tensioner construction for the user to loosen tension on. See pictures. (1st pic is M278. 2nd pic is M273).


M278 2013+ X166 Mercedes Benz Belt Tensioner

M273 2007-2012 X164 Mercedes Benz Belt Tensioner


I didn't think the center bolt of the tensioner pulley would actually be the hex bolt to loosen tension on, but it is. I put the socket on and turned it counterclockwise to loosen. If you look at the tensioner there is no other place to loosen... If I recall, the bolt is a 14mm. Furthermore, I have never heard of the brand "Gates". if you are questioning the integrity of OE "Gates" tensioner. I noticed the old belt tensioner I removed from my 2013 GL550 is also "Gates" with a mercedes logo stamped on the other side.

Regarding the idler pulleys, pay attention to which pulley goes where. They are all distinctively different and all the pulleys come with new screws except the large idler pulley. For the large idler pulley, pay attention to which side is indented and which side is flat when removing.

Here is everything I removed:

 

Thanks for the writeup. Gates is a long time brand name in aftermarket parts. Original company founded in 1911.

https://en.wikipedia.org/wiki/Gates_...nd%20customers.

 

@WinterPark321 Do you recall the pulley torques? I have the work instruction but one of the pulleys calls for 70NM which seems super high. I started a thread asking for the torque but i'm a little confused with the Work Instruction. https://mbworld.org/forums/e-class-w...er-torque.html

 

Just curious, is it advisable to replace all the pulleys? At what mileage?
Do you still have to remove the fan and charge pipes when you just replace the belt and not the pulleys?
Thanks!