Mercedes Chain Rattle

Mercedes M157, M276, & M278 Rattle Noise Upon Starting

Many of the newer generation Mercedes engines (namely the M157 AMG engine, M276 V6 engine, & the M278 V8 engine) develop an obnoxious rattle sound upon start up. This is a common symptom which is already fairly well documented on the internet. The purpose of this article is to make the suggested repair pathway easier to decide upon. 

The noise is essentially chain rattle and/or rattle from the camshaft adjuster units; however, the cause can be any combination of the following: poor oil pressure to the secondary chain tensioners upon start up, defective chain tensioners, defective camshaft adjusters, stretched timing chains, and/or damaged timing chain guide rails. The installation of oil check valves is required for all repair options!

Below I will list each engine code and the suggested repair path from Mercedes-Benz….

M157 AMG engines

For vehicles with engine number 157 9xx … through engine number 157 9xx … install check valves and revised chain tensioners.

For vehicles with engine number 157 9xx … through engine number 157 9xx … install check valves (revised tensioners are already installed in this production range).

M278 V8 engines

For vehicles with engine number up to 278 9xx … install check valves and revised chain tensioners.

For vehicle with engine number 278 9xx … through engine number 278 9xx … install check valves (revised tensioners are already installed in this production range).

M276 8 V6 engines

For vehicles with engine number up to 276 8xx … install check valves and revised chain tensioners.

For vehicles with engine number 276 8xx … through engine number 276 8xx … install check valves only (revised chain tensioners are already installed in this production range).

M276 9 V6 engines

For vehicles with engine number up to 276 9xx … install check valves and revised chain tensioners.

For vehicles with engine number 276 9xx … up to engine number 276 9xx … install check valves only (revised tensioners installed in this production range).

Things to remember and furthermore things to consider when making related repairs…

1. These recommendations are published by Mercedes for dealership technicians whom are generally working on lower mileage vehicles.
2. I have personally seen several failures of the “revised” secondary chain tensioners. I highly recommend installation of the check valves and replacement of the chain tensioner despite the engine number! You have already made them accessible and they are not too costly.
3. It is also ideal to check the timing components for wear such as chain stretch and guide. Chain stretch is something I am starting to see on these engines, along with guide rail breakage. Mercedes has a special tool for measuring wear on the timing components but a digital caliper has thus far been sufficient. Specifications can be found within WIS (Mercedes workshop information system) or on many of the applications within AllData or Mitchell repair information databases.
4. Camshaft adjusters also fail on these engines and can create or enhance rattle noises. Rotating the engine from the crankshaft pulley (of course ensure the timing chain components are all installed) may exhibit a snapping or popping during rotation from the adjuster units.