Abstract: 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.

Abstract: Disclosed is a fuel vapor treating apparatus, in which fuel vapor evaporated in a fuel tank is not released directly into the atmosphere but is collected and treated. This apparatus is provided with a canister for collecting fuel vapor and a purge line for purging the fuel vapor collected in the canister to an intake passage of an engine. The canister has a first control valve for controlling introduction of fuel vapor thereto. The canister also has a second control valve for controlling the introduction of outside air and the exhaust of gas therefrom. The first control valve communicates with a vapor line extending from the fuel tank and is opened whenever the internal pressure of the fuel tank exceeds a predetermined level to introduce fuel vapor from the fuel tank into the canister. The second control valve is designed to communicate with the atmosphere and is opened whenever fuel is purged through the purge line to the intake passage to introduce outside air into the canister.