Method and system for rich condition vapour purge reset based on tank vacuum level condition

A system and method for controlling the purging of vapour from a carbon canister (22) based on identification of a vacuum loss in the manifold. The system and method of the present invention store target values for the tank pressure and purge duty cycle. A differential pressure between current system levels and target levels is calculated and compared to predetermined critical levels. If the currently calculated level exceeds a calibrated threshold value, a countdown timer is set and reset until the calculated differential drops below the threshold value. If the engine air mass drops below a critical calibrated air mass value before the timer has completed counting down, the purge flow is reset and slowly reintroduced to prevent rich engine air/fuel ratio conditions.