Abstract: An improved method of testing for evaporative emission system leaks monitors vacuum decay in a closed system so that the effects of fuel tank expansion during the test interval are minimized. In a first embodiment pass/fail criteria are established in terms of the time required for the system pressure to decay by a calibrated amount for a predetermined leak size. A leak at least as large as the predetermined leak is detected if the measured time is shorter than a calibrated time. The effects of fuel tank expansion are minimized because the changes in fuel tank volume occur primarily due to the pressure differential across the tank, as opposed to the leak size, and the changes that occur during the test are essentially the same for any leak size under consideration.

Abstract: An improved method of testing for evaporative emission system leaks monitors vacuum decay in a closed system so that the effects of fuel tank expansion during the test interval are minimized. In a first embodiment pass/fail criteria are established in terms of the time required for the system pressure to decay by a calibrated amount for a predetermined leak size. A leak at least as large as the predetermined leak is detected if the measured time is shorter than a calibrated time. The effects of fuel tank expansion are minimized because the changes in fuel tank volume occur primarily due to the pressure differential across the tank, as opposed to the leak size, and the changes that occur during the test are essentially the same for any leak size under consideration.