Abnormality diagnostic method for exhaust gas recirculation control system

A control unit (40) for an exhaust gas recirculation control system (30) compares an actual intake air quantity GN with a predetermined quantity GNC1 when an engine (10) is in a stable idling state (steps S100,S110). Then, presuming there is a t endency for the amount of recirculated exhaust gas from the exhaust gas recirculation control system (30) to be less than a reference amount corresponding to the stable idling state, a recirculated exhaust gas insufficient flag XS is set to "ON" (step S140) after the actual intake air quantity GN has exceeded the predetermined quantity GNC1 for a predetermined period of time (steps S120,S130). The control unit (40) th en makes a condition for making a positive diagnosis more difficult to fulfill so that it is more difficult to make a positive diagnosis when the recirculated exhaust gas insufficient flag XS is set to "ON."