Evaporative fuel-processing system for internal combustion engines

An evaporative fuel-processing system for an internal combustion engine having an intake system, and a fuel tank, comprising:

• an evaporative emission control system including a canister having an adsorbent accommodated therein, for adsorbing evaporative fuel generated in said fuel tank, and an air inlet port communicating with atmosphere, a charging passage extending between said canister and said fuel tank, a purging passage extending between said canister and said intake system, a purge control valve arranged across said purging passage, and a vent shut valve for opening and closing said air inlet port of said canister;
• pressure-detecting means for detecting pressure within said evaporative emission control system;
• open-to-atmosphere means for relieving said interior of said evaporative emission control system to said atmosphere, by closing said purging valve and opening said vent shut valve;
• negatively pressurizing means for negatively pressurizing said interior of said evaporative emission control system into a predetermined negatively pressurized state, by opening said purge control valve and closing said vent shut valve;
• leakage-checking means for closing said purge control valve after said interior of said evaporative emission control system is brought into said predetermined negatively pressurized state, and for determining whether there is leakage from said evaporative emission control system, based on a rate of decrease in negative pressure within said evaporative emission control system after the closing of said purge control valve;
• tank state-detecting means for detecting a pressure state within said fuel tank, based on an output from said pressure-detecting means during operation of said open-to-atmosphere means;
• and
• operation-determining means for determining whether said negatively pressurizing means is to be operated, based on a result of the detection by said tank state-detecting means,
• wherein said operation-determining means determines that said negatively pressurizing means is not to be operated, when said tank state-detecting means detects that said pressure within said fuel tank was positive at the start of the operation of said open-to-atmosphere means and at the same time an amount of change in pressure within said fuel tank after the start of the operation of said open-to-atmosphere means exceeds a predetermined value.