Fuel system and method for operating a fuel system

The system (1) has a ventilation system (3) ventilating a fuel tank (2) or pressure tank and comprising a separation device (4) exhibiting an intermediate storage (5) for liquid fuel. The separation device has a filling level determining device (14) for determining a filling level in the storage. The filling level determining device comprises a mechanical and/or electrical filling level sensor (15). A control device is assigned to the level determining device and controls a ventilation valve e.g. overflow valve, or a fuel conveying device i.e. suction jet pump, in dependent upon the level. The mechanical and/or electrical filling level sensor is designed as a capacitive sensor, inductive or resistive sensor, wheel sensor, microwave sensor, optical sensor or fluid sensor. An independent claim is also included for a method for operating a fuel system.