A method for operating and checking the tightness of a
fuel supply system of a motor vehicle with a
fuel tank (1) in which an
overpressure or underpressure measurable by means of a
pressure sensor (9) can prevail, as long as its vent line (6a, 6b) leading through a
fuel vapor intermediate storage tank (8) is sealed off from the environment for fuel components discharged in gaseous form from the
fuel tank (1), wherein a switchable shut-off valve (7a) is provided in this vent line (6a, 6b) between the
fuel tank (1) and the
fuel vapor intermediate storage tank (8), and further switchable shut-off valves (7b, 7c) are provided in a first line (6c, 6d) leading from the
fuel vapor intermediate storage tank (8) to the environment (U) and in a second line (6e) leading from the fuel vapor intermediate
storage tank (8) to a
consumer (2) of the fuel located in the fuel tank, characterized in that - then, when the motor vehicle or at least its
consumer (2) is not in operation, the shut-off valve (7a) provided between the fuel tank (1) and the fuel vapor intermediate storage (8) and the shut-off valve (7c) provided between the fuel vapor intermediate storage (8) and the
consumer (2) are kept closed, while the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is kept open, - and that, starting from the aforementioned condition, a two-stage
verification procedure is initiated at specific times controlled by the decommissioning or commissioning of the motor vehicle or the consumer, - as part of which it is first checked using the
pressure sensor (9) whether there is at least a slight
differential pressure in the fuel tank (1) compared to
ambient pressure, - and that the shut-off valve (7b) provided between the fuel vapor intermediate storage (8) and the environment (U) is then closed. - and the shut-off valve (7a) provided between the fuel vapor intermediate storage (8) and the fuel tank (1) is opened - and the temporal progression of the pressure measured by means of the
pressure sensor (9) is monitored for a certain period of time on the order of one or a few minutes, - where, if, during this inspection procedure, at least a slight
differential pressure was measured before the switching of the shut-off valves (7a, 7b) and this pressure did not change by more than a specified
limit value within a certain period of time after the switching of the shut-off valves (7a, 7b), it is concluded that the
fuel supply system is sufficiently tight, - wherein, in the event that the query performed in a first stage of the
verification procedure as to whether at least a slight
differential pressure exists in the fuel tank is negative, the
verification procedure is restarted after a certain period of time, and wherein, after a predetermined number of unsuccessful attempts to complete the
verification procedure, it is recognized that no verification is currently possible, whereby this conclusion is only made if an exception boundary condition was detected in a query performed in a previous procedure step, whereas, if no exception boundary condition exists, after a predetermined number of unsuccessful attempts to complete the
verification procedure, an area (A) of the
fuel supply system is recognized as "leaking".