Failure diagnosis apparatus for evaporative fuel processing system

A failure diagnosis method for diagnosing a failure of an evaporative fuel processing system that includes a fuel tank, a canister having adsorbent for adsorbing evaporative fuel generated in said fuel tank, an air passage connected to said canister for communicating said canister with the atmosphere, a first passage for connecting said canister and said fuel tank, a second passage for connecting said canister and an intake system of an internal combustion engine, a vent shut valve for opening and closing said air passage, and a purge control valve provided in said second passage, said failure diagnosis method comprising the steps of:

• a) detecting stoppage of said engine;
• b) detecting a pressure in said evaporative fuel processing system;
• c) closing said purge control valve and said vent shut valve when stoppage of said engine is detected,
• d) determining whether there is a leak in said evaporative fuel processing system based on a determination parameter corresponding to a second-order derivative value of the detected pressure during a first predetermined determination period after closing of said purge control valve and said vent shut valve;
• e) determining whether the leak is present in said evaporative fuel processing system based on a relationship between the detected pressure and a staying time period in which the detected pressure stays at a substantially constant value, during a second predetermined determination period which is longer than the first predetermined determination period after closing of said purge control valve and said vent shut valve;
• f) calculating an evaporative fuel parameter indicative of an amount of evaporative fuel generated in said fuel tank after stoppage of said engine;
• and
• g) selecting one of the determination results of said step d) and said step e) according to the calculated evaporative fuel parameter.

A computer program embodied on a computer-readable medium, for causing a computer to carry out a failure diagnosis method for diagnosing a failure of an evaporative fuel processing system that includes a fuel tank, a canister having adsorbent for adsorbing evaporative fuel generated in said fuel tank, an air passage connected to said canister for communicating said canister with the atmosphere, a first passage for connecting said canister and said fuel tank, a second passage for connecting said canister and an intake system of an internal combustion engine, a vent shut valve for opening and closing said air passage, and a purge control valve provided in said second passage, said failure diagnosis method comprising the steps of:

• a) detecting stoppage of said engine;
• b) detecting a pressure in said evaporative fuel processing system;
• c) closing said purge control valve and said vent shut valve when stoppage of said engine is detected,
• d) determining whether there is a leak in said evaporative fuel processing system based on a determination parameter corresponding to a second-order derivative value of the detected pressure during a first predetermined determination period after closing of said purge control valve and said vent shut valve;
• e) determining whether the leak is present in said evaporative fuel processing system based on a relationship between the detected pressure and a staying time period in which the detected pressure stays at a substantially constant value, during a second predetermined determination period which is longer than the first predetermined determination period after closing of said purge control valve and said vent shut valve;
• f) calculating an evaporative fuel parameter indicative of an amount of evaporative fuel generated in said fuel tank after stoppage of said engine;
• and
• g) selecting one of the determination results of said step d) and said step e) according to the calculated evaporative fuel parameter.