An idle speed control system for controlling the idle speed of an internal combustion engine, to conserve fuel by allowing a lowered idle speed, whenever possible, as well as offering more than normal power when needed, is disclosed. This idle speed control system includes an actuator for moving a secondary idle stop member into operative position, having both a vacuum-operated section and a solenoid section for maintaining the actuator in operative position, regardless of the state of the vacuum-operated portion. The idle speed control system further includes a control circuit responsive to engine speed which applies an output signal to the vacuum-operated section of the actuator when engine speed falls below a predetermined minimum, maintains it for a predetermined period of time, momentarily removes it to determine if the engine is presently capable of idling above the predetermined minimum speed, and reapplies the output signal if engine speed then dips below the predetermined minimum speed. Once applied, the output signal is removed either by the timer function, or when engine speed increases above the predetermined maximum idle speed. The control circuit also provides a signal to the solenoid portion of the actuator, in response to predetermined major accessory loads which may be imposed upon the engine in a vehicle.