Auxiliary Air Valve (Idle Speed): Description and Operation

Auxiliary air valve





The auxiliary air valve is mounted on the engine's thermostat housing and acts as a by-pass passage enabling air to flow past the butterfly in the throttle body when the engine is cold.

The auxiliary air valve consists of an aluminium body with a connector for the internal heating coil and spigots for the hoses from the throttle body.Running through the aluminium body is an air passage which is opened and closed by a valve disc operated by the bi-metallic strip.

When the engine is cold, the air passage is fully open. When the engine is started, the heating coil is energized and heats the bi-metallic strip which then gradually closes the valve.

If the engine is hot when started, the heat given off by it acts on the bi-metallic strip and the valve remains almost closed, opening only very slightly.