SELF-DIAGNOSING DEVICE FOR EXHAUST GAS RECIRCULATION SYSTEM FOR INTERNAL COMBUSTION ENGINE

PURPOSE: To perform high-precise self-diagnosis without increasing a cost by determining the opening error of an exhaust gas recirculation control valve according to a difference between a control signal to an idle control valve and a target control signal during idling and outputting an alarm when the opening error exceeds a given value. CONSTITUTION: When decision of an idle rotation control executing condition and a self-diagnosis starting condition is effected and self-diagnosis is not carried out, an ISC duty ratio learnt and updated according to an idle rotation control routine set by an idle rotation control device is set to a target duty ratio. When the number of revolutions of an engine attains the target number of idle revolutions, a target duty ratio is subtracted from a present ISC duty ratio by an opening error estimating means to determine the deviation of an ISC duty ratio. By using the value, an EGR target opening correction table is retrieved to determine the correction value of the target opening value of an EGR control valve. A correction value is compared with a trouble decision threshold by a trouble deciding means and when the correction value exceeds the trouble decision threshold, it is decided that a trouble occurs and an alarm is outputted.