Evaporative fuel control unit for internal combustion engine

Abstract

Disclosed is an evaporative fuel purging apparatus used for an internal combustion engine. A canister having an absorbing agent for absorbing an evaporative fuel generated in a fuel tank of the internal combustion engine is provided. A drain control valve is provided on a drain port for opening the canister to the atmospheric air, and which is adapted to open and close the drain port. A purge control valve is provided on a purge passage communicating the canister to a part of an intake system, and which is adapted to purge the evaporative fuel absorbed into the canister through opening and closing the purge passage. A diagnostic device diagnoses the presence or absence of leakage in a purge system which is sealed up by closing the drain control valve and the purge control valve after pressure-reduction treatment. In the above apparatus, the drain control valve is opened after termination of the diagnosis by the diagnostic device, to open the canister to the atmospheric air through the drain port, and after an elapse of a specified time, the purge is started by opening the purge control valve. With this construction, it is possible to prevent the rapid change in the air-fuel ratio in re-starting the purge after termination of the diagnosis for the purge system, and hence to improve the stabilization of the emission and the drivability.