During idle operation, the throttle valve is almost completely closed. Idle air enters the engine through an adjustable throttle air bypass screw which varies the amount of air which can flow past the closed throttle valve. By turning this screw clockwise, throttle bypass air is reduced, causing a decrease in idle speed. Conversely, turning the screw counterclockwise will increase idle speed by allowing more air to pass the closed throttle valve.
On engines equipped with an ECU controlled ISCV, this throttle air by-pass screw is seated at the factory, and there are no provisions for curb idle adjustment. Idle air is varied by the ECU through control of the ISC Valve position.
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