Evaporative fuel control system for internal combustion engines

An evaporative fuel control system for an internal combustion engine having an intake system, a throttle valve arranged in said intake system, and a fuel tank, comprising:

• a canister for adsorbing evaporative fuel generated in said fuel tank;
• a purging passage extending between said canister and said intake system, for purging evaporative fuel into said intake system at a location downstream of said throttle valve;
• a purge control valve arranged across said purging passage, for controlling a flow rate of evaporative fuel supplied to said intake system through said purging passage;
• and
• operating error-determining means including means operable when said engine is in a predetermined operating stable condition, for calculating a first air-fuel ratio influence degree parameter indicative of a degree of influence of said evaporative fuel on an air-fuel ratio of a mixture supplied to said engine, means for changing said flow rate of said evaporative fuel at a predetermined ratio by controlling said purge control valve, to therein calculate a second air-fuel ratio influence degree parameter indicative of said degree of influence of said evaporative fuel on said air-fuel ratio of said mixture after said changing of said flow rate of said evaporative fuel, and means for determining a value of said flow rate of said evaporative fuel assumed when said first and second influence degree parameters indicate that said degree of influence of said evaporative fuel on said air-fuel ratio of said mixture is zero, based on the calculated first and second air-fuel ratio influence degree parameters, and means for determining a control amount of said purge control valve corresponding to the determined value of said flow rate of said evaporative fuel, and for determining an operating error of said purge control valve, based on the determined control amount.