Have Mercedes Questions? Ask a Mechanic for Answers ASAP
Hi, the best way to verify the issue is with a smoke test of the evap system. Smoke is induced into the closed system and the technician will look for smoke escaping. The valves will not usually cause a fuel smell when the tank is full. Keep in mind that the canister shut off valve is normally open and the purge valve is normally closed. If the valves fail they don't cause an odor they just cause fault codes. It is more likely that you have a fuel or vapor leak at the top of the fuel tank. The rear seat cushion should be removed to inspect the pump and sending unit at the top of the fuel tank on the left and right side.
Let me know if you have any questions.