Exhaust Gas Recirculation: Description and Operation

EGR Valve:

The purpose of the EGR system is to limit the formation of oxides of nitrogen (NOx) by diluting the fresh intake charge with a metered amount of exhaust gas, thereby reducing the peak temperatures of the burning gases in the engine combustion chambers.
The system consists of a diaphragm actuated flow control valve (EGR valve), coolant temperature override (CTO), thermal vacuum switch (TVS), and connecting hoses.
The valve is held in a normally closed position by a coiled spring located above the diaphragm. A special fitting is provided at the carburetor to route ported (above throttle) vacuum through hose connections to a fitting on the valve which is located above the diaphragm. A passage in the intake manifold directs exhaust gas from the exhaust passage to the EGR valve. When the diaphragm is actuated by vacuum, the valve opens and meters exhaust gas through another passage in the intake manifold to the floor of the manifold below the carburetor.

Dual Function CTO:

COOLANT TEMPERATURE OVERRIDE (CTO)
The CTO restricts vacuum flow to the Thermal Vacuum Switch when the coolant temperature is below 115° F. When the coolant temperature reaches 115° F a vacuum signal is applied to the TVS.

EGR Thermal Vacuum Switch (TVS):

THERMAL VACUUM SWITCH (TVS)
The TVS functions as an ON/OFF switch controlled by temperature in the air cleaner. The TVS controls the vacuum passage between the EGR valve and the CTO switch. When the temperature is below 55° F the TVS restricts vacuum to the EGR valve and de-activates the EGR system, thereby improving cold weather performance.