| 摘要 |
PROBLEM TO BE SOLVED: To prevent wide variation of the air-fuel ratio, when a purge valve is opened and inside an evaporated fuel system is to be decompressed in order to make diagnosis of a leakage at the evaporated fuel system. SOLUTION: When failure diagnosis is made at idling, an atmospheric pressure release valve 37 of a canister 30 is closed, a purge valve 35 is opened, and inside an evaporated fuel system is decompressed by the engine suction negative pressure. When a prescribed negative pressure is attained, the purge valve 35 is also closed so that inside the evaporated fuel system is held at negative pressure. When the degree of pressure rise in the condition held at a negative pressure is over the prescribed level, it is considered that a leak exists. The outputs of oxygen sensors 13L and 13R installed in the engine exhaust passage are turned on and off at the boundary consisting of the stoichiometric air-fuel ratio, and using these outputs of oxygen sensors at idling, the fuel amount supplied to the engine is feedback controlled so that the actual air-fuel ratio becomes identical to the stoichiometric ratio. In a decompression process, the purge valve 35 is first put in a certain prescribed degree of opening and then is increased gradually in opening degree synchronously with inversion of the outputs of the oxygen sensors.
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