Abstract: The evaporative emission control system of an internal combustion engine is tested by closing the purge air inlet from the atmosphere, applying a vacuum signal to the system, storing the value of a closed loop air/fuel adjustment at the time the vacuum signal is first applied, sensing for the vacuum signal at a specified point in the system and indicating a fault condition if the vacuum signal is not sensed within a first time period and when the amount of the closed loop air/fuel adjustment at the end of a second time period varies from the stored adjustment by an amount less than a predetermined amount.