The chain can wear. I think the manual has a procedure for measuring stretch. If it is worn, replace it. When I sold my '86 190E, I warned the new owner that it should be replaced soon. He was an old foreign car mechanic. He did not replace the chain. It broke and trashed the engine. A young driver in his household had already trashed the car (how many things can you run into in a short time-not a straight panel left on the car). Car went to salvage yard and parts are now all over the place going to good use.