The gentlemen above have pretty much covered it. David's suggestion is perhpas the first thing you should do. Don't mess with anything, just take it to a German car specialist to get it checked out (aka, getting a free estimate). Then come tell us what they tell you -- to see if it is a DIY job (very likely will be). Man, you are getting pretty bad gas mileage. I get about 25mpg evenly distributing my travel between highway and city.