I don't know if this applies to your situation, but I "almost" screwed up the sending unit in my 1999 C280 by overfilling the gas tank. When I first got the car, I would continue to put more gas in the tank to top it off after the gas nozzle initally clicked off the first time. After a while, my gas gauge started acting erratically. Initially, it would stay on empty after I started the car in the morning, then would move up to the proper position after a few minutes. Finally, it stopped working entirely and would stay on empty. I dreaded removing the gas tank to replace the two sending units for $400. I read online about running fuel injection cleaner through a few tanks of gas. I used Berryman's B12 Cleaner from WalMart and it corrected the problem after 3 or 4 tanks of gas. It sure beat removing the gas tank! The online article mentioned something about residue building up on the sending unit.