Idle Speed is the situation where the engine is operated without depressing the accelerator pedal and the engine is separated from the transmission. During Idle Speed, the engine crankshaft rotates and provides adequate power to drive the engine subsystems. Other vehicle systems not related to engine operation such as the AC system place unnecessary loads on engines. Engines must compensate for these additional loads by increasing the idle speed or else it may stall.
In the past, the engine Idle Speed is controlled by rotating the adjusting screw on the throttle body. Modern vehicles however, are equipped with Idle Control Systems which automatically adjust idle speeds. Data used in Idle Control are provided by several sensors that monitor crankshaft rotations, throttle positions, coolant temperatures and vehicle speed. When Idle Control Sensors sense a drop in engine rpm and the engine is in danger of stalling, the ECU actuates the Idle Control Solenoid to admit extra air into the bypass passageway over the throttle to increase the engine idle speed.
Idle Speed Control is very useful especially when driving in heavy traffic where engine load varies. Faulty Idle Speed Control Sensors cause idle speeds to surge or drop unpredictably. Idle Speed Control Valves are exposed to high temperatures and engine vibrations which might loosen its electrical connections. Most vehicles with damaged ISCSs will not run unless the sensor is serviced or replaced.
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