Patent ID: 7418856

Claim:
Method for checking the gastightness of a motor vehicle tank ventilation system, the system comprising: a fuel tank, an adsorption filter, ventilation line connecting the fuel tank to the adsorption filter, a fresh air line which connects the adsorption filter to the atmosphere and in which a controllable check valve is located as the adsorption filter check valve (AAV), a regeneration line which connects the adsorption filter to the intake manifold of the internal combustion engine and in which there is a controllable regeneration valve as the tank ventilation valve (TEV), at least one pressure sensor in the tank ventilation system, and a control/evaluation unit, to check the tank ventilation system with the adsorption filter check valve (AAV) closed by controlled opening of the tank ventilation valve (TEV), a negative pressure being produced in the tank ventilation system with a pressure variation which is evaluated after controlled closing of the tank ventilation valve (TEV) for tank leak diagnosis, the method comprising: during operation of the internal combustion engine the pressure in the tank ventilation system is set to the ambient pressure level (P 0 ) by controlled closing of the tank ventilation valve (TEV) with the adsorption filter check valve (AAV) opened, at a stable operating point of the internal combustion engine the adsorption filter check valve (AAV) is closed and the tank ventilation valve (TEV) is opened, by which a negative pressure builds up, the negative pressure buildup is determined up to a specified negative pressure buildup threshold (P S2 ) which is detected by at least one pressure sensor, the assigned negative pressure buildup time (t 1 ) is measured, when the negative pressure buildup threshold (P S2 ) is reached the tank ventilation valve (TEV) is closed, by which the negative pressure decays again, the negative pressure decay is determined up to a specified negative pressure decay threshold (P S1 ) which detected by at least one pressure sensor and the assigned negative pressure decay time (t 2 ) is measured, the negative pressure buildup time (t 1 ) and the negative pressure decay time (t 2 ) are added to form the total diagnosis time (t 1 ) and are compared to a diagnosis time threshold (t DS ), when the diagnosis time threshold (t DS ) is not reached a fault signal being produced.