System and method for determining leakage in fuel vapor emission controls

A pressure responsive relief valve (38) is disposed in the tank vapor line to the engine inlet manifold (54) and upstream of an electric flow control valve (50). The pressure responsive valve (38) has a bleed orifice (42) for providing limited flow for leak testing when the relief valve (38) is closed; and, the relief valve (38) is set for opening at a selected pressure differential to provide adequate vapor canister purge flow during engine operation. A differential pressure sensor (60) is employed to read the pressure drop across the bleed orifice (42) during leak testing and the flow determined from a lookup table for comparison with a threshold value to determine if leakage is present in the system.