Idling speed control device of an internal combustion engine

An idling speed control device of an internal combustion engine having an intake passage and a throttle valve arranged in the intake passage, said device comprising:

• a bypass passage interconnecting the intake passage located upstream of the throttle valve to the intake passage located downstream of the throttle valve;
• valve means arranged in said bypass passage and having a valve chamber which has an air inlet opening and an air outlet opening, said air inlet opening being formed on an inner bottom wall of said valve chamber and connected via said bypass passage to the intake passage located upstream of the throttle valve, said valve means having a control valve for controlling a flow area of said air outlet opening which is connected to the intake passage located downstream of the throttle valve, and;
• a stepper motor connected to said control valve for controlling the amount of air flowing within said bypass passage in accordance with a change in the operating condition of the engine at the time of idling, and
• wherein said stepper motor includes a motor housing, a stator stationarily arranged in said motor housing, a rotor, and a valve shaft axially movable in said motor housing and actuated by said rotor, said control valve being fixed onto said valve shaft, and
• wherein said rotor includes a hollow cylindrical outer body made of a permanent magnet, a hollow cylindrical inner body made of a synthetic resin and rotably mounted on said valve shaft and a hollow cylindrical intermediate body interposed between said hollow cylindrical inner and outer bodies and made of a metallic material, said hollow cylindrical intermediate body being supported on said motor housing by means of bearings.