Some 2002-11 Mercedes G55, G500 and G550 SUVs may leak fuel from the fuel tank's filler neck area. In some cases, the Check Engine light may be illuminated with EVAP system trouble codes stored in PCM memory. One likely cause for the trouble, reports Mercedes, is an incorrectly installed bleed hose to the switchover valve. The condition typically occurs after the fuel tank has been dropped for service.
To make the diagnosis, look at the position of the bleed hose end in relation to the switchover valve. When properly installed, there should be a gap of .200" between the end of the hose and the valve. No gap puts excess strain on the switchover valve, which can cause it to crack. That's what causes the EVAP codes to log in computer memory. If the valve is cracked, you have no alternative but to replace the entire filler neck (the switchover valve isn't available separately).
If the valve is in good shape and you have a fuel leak, just reposition the hose so that the .200" gap is restored.