Thermostatic Air Cleaner: Description and Operation

Thermostatically Controlled Air Cleaner Assembly:

SYSTEM OPERATION

This system is part of the air cleaner assembly, and its main purpose is to reduce hydrocarbon (HC) emissions and improve cold driveability. 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 running characteristics. This system maintains this predetermined temperature through the use of manifold vacuum, an Inlet Air Temperature Compensator (ITC) valve, hot air that radiates off the exhaust manifold, and a vacuum diaphragm that operates the hot air control valve in the air cleaner snorkel.

When the engine is "OFF," the hot air control valve is closed so that no hot air can circulate to the air cleaner. When the engine is started in a cold ambient environment, the ITC valve opens allowing manifold vacuum to flow to the vacuum diaphragm. Once manifold vacuum is applied to the vacuum diaphragm, the hot air control valve opens which closes the ambient air duct and opens the passage to the hot air duct. The TCA system is overridden if engine speed is increased rapidly during cold engine operation. This occurs because as the engine speed is accelerated, there is a drop in manifold vacuum which allows the spring force in the vacuum diaphragm to move the hot air control valve, closing the hot air duct.

Under normal engine operating conditions, the purpose of the TCA system is to switch the hot air control valve so that ambient air is consumed by the engine instead of heated air.

Under extreme conditions (climbing a steep grade, running at high speeds for extended periods of time, or extended idling) engine and engine compartment temperatures significantly increase. This increase in temperature will cause excessive fuel vapors to enter the intake manifold, which causes rough idle and excessive emissions. The TCA system prevents this situation by allowing an additional amount of air to be supplied to the intake manifold via the air cleaner, the ITC valve, and its vacuum hose.
Under these extreme conditions, the ambient air passage remains open to allow fresh air to the engine. This passage is open because their is a low vacuum level to the vacuum diaphragm, as in normal engine operating conditions.

ITC Valve:

INLET AIR TEMPERATURE COMPENSATOR (ITC) VALVE

This valve is the main controlling component of the TCA system. It directs vacuum to the vacuum diaphragm under either normal or cold start conditions, and it directs fresh air from the air cleaner to the intake manifold when engine and engine compartment temperatures get too high. The main functions of this valve are:

1. To close "OFF" the source of high (manifold) vacuum to the vacuum diaphragm, and open the passage of low (air cleaner) vacuum to the vacuum diaphragm under normal operating conditions.

2. To close "OFF" the source of low (air cleaner) vacuum to the vacuum diaphragm, and open the passage of high (manifold) vacuum to the vacuum diaphragm under cold start conditions.

3. To close "OFF" the source of high (manifold) vacuum to the vacuum diaphragm and allow fresh air to circulate from the air cleaner, through the ITC valve, and out to the intake manifold under extreme engine conditions described in the SYSTEM OPERATION.