Thermostatic Air Cleaner: Description and Operation

OPERATION

This system is part of the air cleaner assembly, and its main purpose is to reduce hydrocarbon (HC) emissions and prevent intake plenum icing. It does this by maintaining a certain temperature inside the air cleaner assembly. By maintaining this temperature, the air/fuel mixture can be adjusted much leaner to reduce emissions and help cold starting characteristics. This system maintains this predetermined temperature through the use of manifold vacuum, a temperature sensor, a heat stove that wraps around the exhaust manifold, a heated air tube, and a vacuum motor operated air control valve.

TAC Control Valve (Heat Off Position):

The air valve in the air cleaner is used to control and insure that the proper ratio of ambient air to heated air is maintained so that the appropriate temperature is obtained inside the air cleaner.

This system has three modes of operation; cold delivery mode, warm delivery mode, and air regulating (mixing) mode. When the ambient temperature is greater than the temperature of air radiating from the exhaust manifold, the air control valve will allow ambient air to flow to the intake plenum while closing off the source of exhaust stove air.

TAC Control Valve (Heat On Position):

When ambient air temperature is below the temperature of the exhaust stove air, then the air control valve will allow air from the exhaust stove to flow into the intake plenum while shutting off ambient air until the engine warms up.

When the ambient air temperature is between these temperatures there will be air flow from both ambient air and the exhaust manifold after the engine has started and the exhaust manifold starts to radiate heat. The colder the ambient air temperatures, the greater the flow from the exhaust stove. The warmer the ambient temperatures, the greater the air flow from the outside air.

Canister Purge Hose Routing:

TEMPERATURE SENSOR

The temperature sensor controls the point at which the air control valve changes positions. It does this by changing the amount of vacuum going to the vacuum motor. When the ambient temperature is below the calibrated opening point of the sensor, full vacuum is applied to the vacuum motor which moves the air control valve to close the ambient air inlet and only allow heated air to enter the air cleaner. As the exhaust manifold temperature and the air temperature inside the air cleaner raises, the temperature sensor senses the change and begins to allow a portion of vacuum to vent to the atmosphere. As vacuum is bled off, the air control valve will move causing the ambient air inlet to open allowing ambient air to enter the air cleaner. As the engine warms up to normal operating temperature, the sensor will bleed more and more vacuum to the atmosphere until no vacuum is flowing to the vacuum motor and the hot air inlet is completely closed.