Exhaust Gas Recirculation: Description and Operation
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.
Fig. 18 EGR valve with integral back-pressure sensor:
Fig. 19 EGR valve without integral back-pressure sensor:
EGR VALVE
The EGR valve, Figs. 18 and 19, is mounted on a machined surface at the rear of the intake manifold on V8 engines, on the side of the intake manifold on six cylinder engines and on a space-plate beneath the carburetor on four cylinder engines. The valve used with an automatic transmission is calibrated differently than the valve used with a manual transmission.
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 crossover passage or from below the riser area 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.
The EGR valve with an integral exhaust back pressure sensor, Fig. 18, combines the functions of the EGR valve and separate back pressure sensor into a single unit. This unit is calibrated by the use of selective diaphragm spring loads and flow control orifices. The exhaust gas flow is controlled by a moveable pintle. When no vacuum is applied to the diaphragm, the pintle is held against its seat by spring tension, confining exhaust gases to the exhaust manifold. When vacuum is applied to the diaphragm, the pintle is forced from its seat, however, this cannot occur if the vacuum bleed valve in the power diaphragm is opened. Exhaust gas back pressure from inside the exhaust manifold flows through the hollow pintle stem into the control diaphragm chamber. If this pressure is enough to overcome the control spring tension, the control diaphragm moves against its bleed valve. Full vacuum is now applied to the power diaphragm and the pintle moves, allowing exhaust gas to flow to the intake manifold. If the exhaust back pressure drops enough, the control diaphragm moves away from the bleed valve and the pintle returns to its seat.
COOLANT TEMPERATURE OVERRIDE SWITCH
This switch is located at the coolant passage of the intake manifold (adjacent to oil filler tube) on a V8 engine, at the left side of the engine block (formerly the drain plug location) on a six cylinder engine or in coolant passage at right rear of cylinder head on four cylinder engines. The outer part of the switch is open and not used. The inner port is connected by a hose to the EGR fitting at the carburetor. The outer port is connected to the EGR valve, or to the TVS on four cylinders.
When the coolant temperature is below 115° F (160° F on some vehicles), the center port of the switch is closed and no vacuum signal is applied to the EGR valve, therefore, no exhaust gas will flow through the valve. When the coolant temperature reaches 115° F (160° F on some vehicles), both the center and inner port of the switch are open and a vacuum signal is applied to the EGR valve. However, the vacuum signal to the EGR valve is subject to regulation by low and high temperature signal modulators.
LOW TEMPERATURE VACUUM SIGNAL MODULATOR
This unit is located at the left side of the front upper crossmember, just ahead of the radiator, on the same mounting bracket as the TCS ambient temperature override switch and is connected to the EGR vacuum signal hose. The modulator is open when ambient temperatures are below 60° F. This causes a weakened vacuum signal to the EGR valve and a resultant decrease in the amount of exhaust gas being recirculated.
HIGH TEMPERATURE VACUUM SIGNAL MODULATOR
This unit is located at the rear of the engine compartment and is connected to the EGR vacuum signal hose. The modulator opens when the underhood air temperature reaches 115° F and causes a weakened vacuum signal to the EGR valve. As a result, the amount of exhaust gas being recirculated is decreased.
Fig. 20 EGR dump valve:
EGR DUMP VALVE
This valve, Fig. 20, is used on some vehicles and is connected between the EGR vacuum signal source and the EGR valve. It prevents EGR operation when intake manifold vacuum drops to 3-5 inches Hg, by ``dumping'' the EGR vacuum signal.
THERMAL VACUUM SWITCH
The Thermal Vacuum Switch (TVS), used on some models, is located in the air cleaner and 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 reaches below 40° to 50° F, the TVS restricts vacuum to the EGR valve and deactivates the EGR system, thereby improving cold weather performance.
FORWARD DELAY VALVE
This valve, used on some models, is installed between the CTO switch and EGR valve and functions to delay the total force of the vacuum applied to the EGR valve. By gradually applying vacuum to the EGR valve, a sudden activation of the EGR system is avoided. When installing a new delay valve, position the black painted side toward the EGR vacuum source.
REVERSE DELAY VALVE
This valve, used on some models, is installed between the CTO switch and the manifold vacuum source. This valve functions to retain the manifold vacuum applied to the distributor vacuum advance mechanism when the engine is cold and is rapidly accelerated. This action prevents the ignition timing from retarding. When the engine coolant reaches a specified temperature, the CTO switch will go to ported vacuum.