ABNORMALITY DIAGNOSTIC METHOD FOR INTERNAL COMBUSTION ENGINE AND ABNORMALITY DIAGNOSTIC DEVICE FOR INTERNAL COMBUSTION ENGINE

During a predetermined operation state in which the opening degree of a throttle valve is fixed, an air flow meter positioned more on the upstream side than a pressure control valve detects a first intake air amount when the opening degree of the pressure control valve positioned on the upstream side of the throttle valve is set to a predetermined first valve opening degree and a second intake air amount when the opening degree of the pressure control valve is set to a predetermined second valve opening degree smaller than the first valve opening degree. On the basis of the first intake air amount and the second intake air amount, a diagnosis is made regarding whether there is an abnormality in a first pipe, a second pipe, a third pipe and the like which are included in a blow-by gas recirculation system for blow-by gas treatment.