A method of operating a fuel system

In a fuel system (10) of an internal combustion engine depends on the injected into a combustion chamber fuel quantity by a control period with which a fuel injection device (22) is driven. A pressure sensor (20) detects the pressure in the fuel rail (16) and provides a signal. The pressure in the fuel rail (16) can be regulated to a desired pressure by means of an adjusting device (18) and using the provided by the pressure sensor (20) signal. For testing of the fuel system (10) is proposed the following method: (A) establishing a einzupritzenden test fuel quantity, (B) operating the internal combustion engine with a first set pressure and the latter and the test injected fuel amount corresponding first control period, (C) detecting the operating state of characterizing, speed or torque-dependent value, (D) operating the internal combustion engine with a second set pressure and the latter and the test injected fuel amount corresponding second activation duration, (E) detecting the operating state of characterizing, speed and torque-dependent variable, (F) performing an action, if the two sizes detected in (c) and (e) differ by more than a threshold value.