Abstract: The present invention provides a diagnostic apparatus for an evaporative emission control system which can accurately execute leak diagnosis under a stable condition. An ECU clocks a time td2 until the internal pressure value Pte of the evaporative emission control system sealed at the negative pressure reaches the target internal pressure value Pd, sets the pressure change measuring time te2 based on the clocking time td2, and measures the pressure change amount P1 which varies after the internal pressure value Pte of the evaporative emission control system reaches the target internal pressure value Pd for a pressure change measuring time te2. Then, the ECU sets the determination value Pth0 based at least on the spatial capacity of the evaporative emission control system, and determines whether the evaporative emission control system is normal or abnormal based on the pressure change amount P1.

Abstract: The present invention provides a diagnostic apparatus for an evaporative emission control system which can accurately execute leak diagnosis under a stable condition. An ECU clocks a time td2 until the internal pressure value Pte of the evaporative emission control system sealed at the negative pressure reaches the target internal pressure value Pd, sets the pressure change measuring time te2 based on the clocking time td2, and measures the pressure change amount P1 which varies after the internal pressure value Pte of the evaporative emission control system reaches the target internal pressure value Pd for a pressure change measuring time te2. Then, the ECU sets the determination value Pth0 based at least on the spatial capacity of the evaporative emission control system, and determines whether the evaporative emission control system is normal or abnormal based on the pressure change amount P1.

Abstract: In order to achieve a correct and efficient diagnosis on an engine, it is important to search a steady state of engine operation correctly and frequently as far as possible. In this invention the steady state operation is judged by checking whether or not the average value of an engine parameter for a predetermined monitoring period is within a predetermined restriction width, whereby so many chances of monitoring for diagnosis can be provided.

Abstract: In a detecting method for deterioration of a catalyst inserted in an exhaust pipe, first, counting the number of times to cut the predetermined slice levels set in accordance with the engine operating conditions by O.sub.2 -sensors provided at the front and rear portions of the catalyst respectively. Then, a ratio of the counted numbers is calculated to judge a deterioration of a catalyst of a catalytic converter. Thus, influence of an output amplitude fluctuation of the RO.sub.2 sensor varying according to the operating condition is avoided to prevent a misjudgement of the deterioration of the catalyst.