Abstract: A fault diagnosis device and process for an exhaust gas recycle control unit of an engine, comprising a return pipe for returning the exhaust gas to an intake pipe, apparatus for opening and closing the return pipe and apparatus for detecting and storing engine operating conditions when the return pipe is opened and closed. The opening and closing apparatus is controlled so that the flow rate of the returned gas is gradually changed when the return pipe is closed from the open state or vice versa.

Abstract: An improved idle speed control apparatus for an internal combustion engine capable of effectively preventing instantaneous fluctuations in the number of revolutions per minute of the engine when the engine load is instantaneously changed during engine idling, thereby improving the driver's driving sensation. A bypass passage is connected with an intake manifold of the engine for supplying intake air to the engine while bypassing a throttle valve in the intake manifold. A bypass air controller is disposed on the bypass passage for controlling the amount of bypass air flowing therethrough. In one embodiment, the bypass air controller operates such that when the engine load is altered instantaneously during the engine idling, the amount of intake air is gradually changed from a first level suited to a first operating condition of the engine load to a second level suited to a second opeating condition of the engine load.