Air Cleaner - Thermostatic: Operation
When engine is cold, air is selected from exhaust manifold shroud, because the heat sensor in air cleaner is cold. In the "open" position, vacuum applied to vacuum motor operates duct valve. See Fig 2 . Duct valve shuts off fresh air supply and opens, allowing heated air to enter air cleaner.
As engine warms up, the sensor operates, preventing vacuum from being applied to vacuum motor. In this "closed" position, the duct valve closes off supply of heated air, and allows air from outside to flow through pickup tube into air cleaner. See Fig 2 .
A cold-weather modulator on some models, controls operation of duct valve under certain air temperature conditions for improved emission control.