Changing the timing chain on a m119 engine is a much more expensive and involved venture. As you will see when you search this forum or other MB forums for more information regarding this matter, there are various schools of thought about "when to change" the timing chain on a M119. If you are concerned, you should have your mechanic measure for chain stretch. (Assuming no one has already replaced the chain tensioner.) The M119 uses a double row chain, but the chain length is quite long. The upper chain guide rails will get brittle over time and crumble. When the covers are removed to check chain stretch, have the upper guild rails checked.