I would start with replacing the sealing sleeve below the cap. Even if the cap seal is leaking the sleeve should prevent gas entering the trunk.

A leak in the fuel system itself (AHA valve and fuel pressure regulator) give a continuous fuel smell if the engine is running.

The w126 has a fuel evaporation system to catch fuel vapors. It releases the pressure, but only with an engine temperature above 50C and throttle valve above idle speed. So if driving short distances fuel pressure in the tank may build up.


Rob