Idling speed control device of an internal combustion engine

An idling speed control device for 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 control valve controlling a flow area of said bypass passage, said valve means having a valve housing and hot fluid passage means, formed in said valve housing, for conducting a heated fluid;
• 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 said engine at the time of idling, said step motor comprising:
• a motor housing,
• a rotor rotatably arranged in said motor housing, said rotor including a hollow cylindrical outer body made of a permanent magnet, said hollow cylindrical outer body having an outer circumferential wall on which an N pole and an S pole are alternately formed, a hollow cylindrical intermediate body disposed concentrically within said outer body and made of a metallic material, said intermediate body being supported on said motor housing by means of bearings, and a hollow cylindrical inner body made of a synthetic resin and disposed concentrically within said intermediate body, said inner body having a center hole in which internal thread screws are formed, and
• a valve shaft axially movable in said motor housing, said control valve being fixed onto said valve shaft, said valve shaft having external screw threads which are in rotatable engagement with internal screw threads on said inner body.