Exhaust Emissions System - Thermostatic Air Cleaners: Operation

With engine temperature below 100°F (38°C), the bleed valve is closed as manifold vacuum builds. The air control diaphragm exerts vacuum pressure which pulls up on the valve door. The valve door rises, permitting heated air from around the exhaust manifold to enter the air cleaner (outside, cold air is blocked).

With engine temperature above 100°F (38°C), the bleed valve is open and manifold pressure bleeds out. The air control diaphragm no longer has vacuum pressure and the internal spring pushes down on the valve door. The valve door returns to down position, blocking off heated air and permitting outside air to enter air cleaner.

A check valve is used to prevent vacuum pressure loss during periods of wide-open throttle. A fixed orifice prevents rapid pressure changes at the diaphragm. It allows smooth operation of the valve door. It also maintains the proper air/fuel ratio.