Hi and thanks everyone for your input. I am not having any problems with my engine now but I was told I would need this major check up at 105,000 miles that what I was checking on. As far as the tensioneris concerned, I changed the complete system because my mechanic said that shock was damaged and there was no more adjustment left. All I needed was the shock but I opted to do the whole thing at once and get it over with. I took it in because there was a kind of banging/rattling noise everytime I stopped at a light. Prior to getting the work done anything major, I get two opinions even if I trust the first one, you never know) I went to a foreign car specialist that gives free estimates just to verify that was the problem. It was the problem and they wanted $ 700 bucksincluding parts!! I purchased the Serpentine belt, the shock and the complete Tensioner kit for $ 235 dollars. My mechanic did the install for $ 150 so my total was $ 385. Mercedes wanted$ 400 dollars without the shock!! They are insane!! Hope this helps. I still have one unanswered question, for this service should I go berserk and let Mercedes this service. I don't think any major service has been done other than oil and filter by the previous owner. May MB would do a better job in this instance because they would be looking for specific things as it is one of theirs. Any help, thank you again!!