Patent ID: 7131322

Claim:
A diagnostic method for an evaporative emission control system of an automotive vehicle, the system in fluid communication with emissions from a fuel tank, an engine, a carbon canister, and atmosphere, the method comprising: monitoring flow of emissions through a purge valve that is located between the engine and the canister, wherein the purge valve is operable to modulate between a fully open and a fully closed position; determining a first ratio of burnable impurities in the emissions; indicating that the purge valve is functioning properly if the first ratio is above a first threshold; determining if flow of emissions through the purge valve is above a second threshold if the first ratio is not above the first threshold; generating a signal that is indicative of a position of a switch located between the carbon canister and the atmosphere, wherein the position is one of open and closed when the evaporative emission control system is in one of a high and low negative pressure condition; monitoring the signal to determine an amount of time that the evaporative emission control system is in the high negative pressure condition for a first period; calculating a ratio of time that the evaporative emission control system is in the high negative pressure condition during the first period; determining if the ratio of time is above a second threshold; indicating that the purge valve is functioning properly if the ratio of time is above the second threshold; and indicating that the purge valve is not functioning properly if the ratio of time is not above the second threshold.