Diagnostic device for an evaporative emission control system

摘要

In an evaporative emission control system, a canister adsorbs fuel vapor sent from a fuel tank of an engine. The canister is connected to an intake air passage through a purge gas passage and a purge control valve. A control circuit employs a pressure sensor to detect the pressure of the canister when the purge control valve is closed and when the pressure of the canister is stabilized. The control circuit carries out a failure diagnosis based on this pressure. Namely, when the pressure of the canister after it is stabilized deviates from the atmospheric pressure by more than a predetermined reference value, the control circuit determines that the canister has failed, i.e., the canister has a leak. Since the diagnosis can be carried out before starting the engine, the diagnosis can be correctly carried out without affecting the operation of the engine.