Evaporative emission control system for internal combustion engines

Claim 1

In an evaporative emission control system for an internal combustion engine having a fuel tank, an intake passage, a throttle valve arranged in said intake passage, an exhaust passage, an exhaust gas component concentration sensor arranged in said exhaust passage for detecting concentration of a component in exhaust gases emitted from said engine, air-fuel ratio control means for controlling an air-fuel ratio of a mixture supplied to said engine by the use of an air-fuel ratio correction coefficient determined in response to an output from said exhaust gas component concentration sensor, said evaporative emission control system including a canister for adsorbing evaporative fuel generated from said fuel tank, a purging passage connecting between said canister and said intake passage at a location downstream of said throttle valve, and a purge control valve arranged in said purging passage for controlling a flow rate of purged gases containing evaporative fuel to be supplied from said canister via said purging passage into said intake passage, the improvement comprising determining means for determining whether said evaporative fuel emission control system is normal or abnormal, from a change in a value of a parameter depending upon said output from said exhaust gas component concentration sensor detected when an opening of said purge control valve is changed and a change in pressure within said intake passage at a location downstream of said throttle valve when said opening of said purge control valve is changed.