Abstract: An improved diagnosis apparatus for detecting leakage of a fuel vapor purge system that purges fuel vapor from a fuel tank to an intake passage of an engine. The apparatus includes a pressure sensor and a purge valve. The pressure sensor detects the pressure in the purge system. The purge valve connects the purge system with the intake passage for lowering the purge system pressure to a predetermined pressure level. After the purge system pressure is lowered to the predetermined level, the purge system is sealed. The apparatus measures the rate of pressure change immediately after the purge system is sealed. The apparatus subsequently measures the rate of pressure change when the purge system pressure reaches a second reference pressure value and computes the ratio of the rates. The apparatus diagnoses whether there is a leak in the purge system based on the ratio and the rate of pressure change at the second reference pressure value.

Abstract: A secondary air supplier supplies secondary air to an exhaust passage, when an engine is in a predetermined condition. An oxygen sensor detects the air-fuel ratio (A/F) in the exhaust gas. The air-fuel ratio indicates either a rich condition or a lean condition. When the engine is cold, a time counter measures a lean period of time, while secondary air is supplied. When the lean period exceeds a predetermined period of time, a diagnostic system determines that the secondary air supplier is functioning properly. If the oxygen sensor detects that the ratio (A/F) indicates a rich condition during the diagnosis, then the time counter stops measuring the lean period, with keeping the total measured lean period. When the engine is hot, the ratio of the lean period to a predetermined period for the secondary air supply could be smaller than a predetermined ratio, and the diagnostic system determines that the air supplier is malfunctioning.

Abstract: An engine controller executes the feedback control of an amount of the fuel injected, such that an air-fuel ratio (A/F) of the exhaust gas approaches a theoretical ratio. The amount of the fuel injected is as a function of a changeable feedback correction factor (FAF). When the engine is in a predetermined condition in which the engine controller interrupts the feedback control, a secondary air supplier supplies secondary air from an intake passage to an exhaust passage, and changes the A/F ratio of the exhaust gas. Furthermore, when the engine controller executes the feedback control operation, the secondary air is temporarily supplied to the exhaust passage, for the purpose of diagnosis of the supplier. The diagnostic system detects a first feedback correction factor before the secondary air is supplied, and a second feedback correction factor after the secondary air has been supplied, and calculates a deviation value therebetween.