Air Cleaner - Thermostatic: Operation
During cold engine operation, below 100°F (38°C), bleed valve is closed. Manifold vacuum builds to allow air control diaphragm to exert vacuum pressure which pulls up on valve door. Valve door rises, permitting heated air from around exhaust manifold to enter air cleaner (outside, cold air is blocked).
During normal engine operation, above 100°F (38°C), bleed valve is open and manifold vacuum bleeds out. Air control diaphragm no longer has vacuum pressure and internal spring pushes down on valve door. Valve door returns to down position, blocking off heated air and permitting cooler outside air to enter air cleaner. A check valve is used to prevent vacuum pressure loss during periods of wide-open throttle operation. A fixed orifice prevents rapid pressure changes at diaphragm. It also allows smooth operation of valve door.