why is my fuel backing up when pumping gas? on 2007 Mercedes-Benz S550

when pumping gas the fuel backs out and the pump keeps clicking off. It takes about 20 minutes to pump a full tank

Asked by for the 2007 Mercedes-Benz S550
Charcoal canister could be soaked with fuel. The "check engine" warning lamp could come on with this also. The canister can be taken off and drained by a technician. Its a good practice to stop putting fuel in tank when the nozzle clicks off the first time. If you keep trying to top off tank as much as you can, the fuel vapor system can get flooded.