I'm hearing a noise from the belt tensioner. Planning to change the idler pulleys too.
Any advice before I start the work next week?
C250 2013 145k miles.

Be careful not to overtighten, you’ll crack the aluminum. Smartest thing to do would be to blow the threads out with compressed air before reinstalling but I personally never have done that.

Check your new upper idler pulley very carefully. I replaced mine with an INA and the one I got out of the box was broken a week later, not sure why but I suspect it was damaged in shipping.

Pay close attention when removing the pulleys. On mine there is a washer installed on the lower idler pulley, between the pulley and the engine. Yours may or may not have it.

Follow the torque specs when installing the new parts. Not sure what engine you have. On the M272 there are two torque specs for the lower idler pulley, one for new installation and one for follow-up operations. The new installation is if the water pump is replaced since the pulley is bolted to it.

Save the old belt. I keep my old belt in the trunk just in case the new one ever breaks while driving.

Totally agree with this. The block material is quite soft and the chance of cross threading or overtightening because of gunk in the threads is high. If your so inclined, a tap and die set will help you clean thr threads. Maks sure you watch bolt length carefully and put them all back in the right spots according to length.

ALL at the kit once us the only way to go!
Serp + Tens. + Idlers

While you're in there manually check individual accesories pulleys for free play/wobbling.

Double check parts in particular the new belt reference may/may not match your engine.

Lots of poor matches found in belt X-Ref tables.

Your original Ref.# is what works right (+/- half Inch Max).

Parts retailers will set you up with 2 or 3 extra inches... waste of time!

Easy job with quality parts.

Thank for all these great advice.
Looked around and found the pulley torque "35 Nm which is 25.8 ft-lb", and for tensioner bolt torque is "25 Nm (18.4 ft-ibs)". can someone confirm this?

This is my Engine no, that means it's a M271 engine?

Engine no.:271860 30 521210

yes it is a M271.860
✌️

The new tensioner did not come with a new bolt.
Do you think I should reuse it or get a new one from the deale?

You’re fine to reuse it unless you’re concerned about it being too rusty or rounded off or anything. The only bolt I replaced was the upper idler pulley and that’s just because it came with one.

not a problem everything is screwed finger tight into aluminum - Go easy!

Even VVT cover fasteners themselves are made of aluminum.