Method of detecting abnormality in fuel supply systems of internal combustion engines

A method of detecting abnormality in a fuel supply system for supplying fuel to an internal combustion engine having at least one exhaust pipe, and an exhaust gas component concentration sensor arranged in each of said at least one exhaust pipe for detecting concentration of a component of exhaust gases emitted from said engine, wherein an amount of fuel supplied to said engine is controlled in a feedback manner based on an air-fuel ratio correction coefficient set in response to an output signal from said exhaust gas component concentration sensor, the method comprising the steps of:

• (1) calculating an abnormality determination value based on said air-fuel ratio correction coefficient;
• (2) calculating a learned average value of said air-fuel ratio correction coefficient;
• (3) renewing said abnormality determination value when said calculated learned average value of said air-fuel ratio correction coefficient falls outside a first predetermined range defined based upon said abnormality determination coefficient;
• and
• (4) determining that said fuel supply system is abnormal when the renewed value of said abnormality determination value falls outside a second predetermined range defined by predetermined upper and lower limit values.