Timing Off?
 

Hi, I own a 1988 300CE and it idles somewhat bad. It isn't real bad but it annoys me sometimes. It also adles bad in reverse. I was wondering if it was the timing chain or tensioner. Or Is it my transmission. I dont think it it my car only has 74,000 miles on it. Please help!

Bad idling can be caused by a host of issues: bad valve stem seals, problems with the fuel system, problems with the fuel injection system, etc. The infamous OVP might also be something to check out ...

If it runs okay off idle it may simply be a dirty throttle body.

How would i clean the throttle body?

With use, the bore gets a buildup of carbon around the butterfly valve which messes up the idle. To clean, remove the throttle body and clean the bore and butterfly valve using carb cleaner (or throttle body cleaning spray) and a toothbrush. You'll have to work at it...the stuff is hard to get off. When clean, you should see an even gap of air around the butterfly valve.