You should always replace all of the o-rings when servicing the fuel filter. Then tighten the filter with a suitable socket until the bolt almost stops turning but not over tight. If it is too loose it will suck air into the filter and the engine will be difficult to start.

------------------
Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM