Abnormality diagnosis apparatus for engine control system

In a condition where an engine is running, a fuel injector injects no fuel, and a fuel pump discharges no fuel so that a fuel pressure in a common-rail is kept constant, an ECU computes an actual torque based on a current engine speed and determines whether the actual torque is increased or decreased relative to a predetermined reference torque. Moreover, in the same condition as above, a fuel pressure in the common-rail is detected. Based on variations of the actual torque and the fuel pressure in the common-rail, the ECU determines whether an abnormality occurs and identifies a type of the abnormality.