I smell gas fumes while running and also while sitting cold

thanx bbmw,so you don't think its the fuel injector and how hard is it to change the things under the back seat? Can a novice like me do it maybe.

I've never seen a Mercedes fuel injector leak. But then, I don't know what's been done to your car or who has done it, so anything is possible. IF you only smell it only when the tank is full, THEN it must be something near the top of the fuel tank (or possibly an unusual malfunction in the EVAP system). I just suggested what i consider to be a very "likely" cause. In my personal opinion, anything that involves fuel and electricity should be left to professionals in controlled environments. Additionally, the locking mechanism that holds those assemblies in place can pretty tricky. It requires a special tool and hands that have done it before. Virtually nothing on the car can be done without a good knowledge base. Even changing the oil requires a special tool. To be on the safe side, I just dug through the TSB list and I don't see any Technical service bulletins or recalls regarding fuel smells. There is one about "musty" odors coming through the AC vents though. But, if you think it smells like gasoline, you should regard it as dangerous and treat it accordingly.