Exhaust Emissions System - Thermostatic Air Cleaners: Operation

When air cleaner is cold the vacuum motor diaphragm is pulled by intake manifold vacuum and opens the hot air control valve. This allows intake of hot air from a stove on exhaust manifold. When the air cleaner reaches a temperature of 100-111°F (38-44°C), the thermo-sensor comes into function and stops the supply of vacuum from the intake manifold.

When vacuum is stopped, the hot air control valve opens and permits entry of cool outside air. This holds the temperature of intake air at a nearly constant temperature. As negative pressure in the intake manifold decreases (closing throttle valves) the vacuum motor diaphragm returns to its normal state and permits entry of cool air into the carburetor even when thermo-sensor is out of function.

The air/fuel mixture tends to richen when engine is running at idle during periods of high operating temperatures. To offset these high temperatures, a hot idle compensator feeds cool ambient air into intake manifold to lean the air/fuel mixture.