DESCRIPTION AND OPERATION
The EGR system consists of a diaphragm actuated flow control valve (EGR valve), coolant temperature override switch, low temperature vacuum signal modulator, high temperature vacuum signal modulator.
All 1977 and later California units have a back pressure sensor which modulates EGR signal vacuum according to the rise or fall of exhaust pressure in the manifold. A restrictor plate is not used in these applications.
The purpose of the EGR system is to limit the formation of nitrogen oxides by diluting the fresh air intake charge with a metered amount of exhaust gas, thereby reducing the peak temperatures of the burning gases in the combustion chambers.
See Figures 1 through 8
The EGR valve is mounted on a machined surface at the rear of the intake manifold on the V8s and on the side of the intake manifold on the sixes.
The valve is held in a normally closed position by a coil spring located above the diaphragm. A special fitting is provided at the carburetor to route ported (above the throttle plates) vacuum through hose connections to a fitting located above the diaphragm on the valve. A passage in the intake manifold directs exhaust gas from the exhaust crossover passage (V8) or from below the riser area (Sixes) 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 intake manifold below the carburetor.
Coolant Temperature Override Switch
This switch is located in the intake manifold at the coolant passage adjacent to the oil filler tube on the V8s or at the left side of the engine block (formerly the drain plug) on the Sixes. The outer port of the switch is open and not used. The inner port is connected by a host to the EGR fitting at the carburetor. The center port is connected to the EGR valve. When coolant temperature is below 115ºF (160ºF on the 8-304 with manual transmission), 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 115ºF, both the center port and the inner port of the switch are open and a vacuum signal is applied to the EGR valve. This vacuum signal is, however, subject to regulation by the low and high temperature signal modulators.
Low Temperature Vacuum Signal Modulator
This unit is located just to the right of the radiator behind the grill opening. The low temperature vacuum signal modulator vacuum hose is connected by a plastic T-fitting to the EGR vacuum signal hose. The modulator is open when ambient temperatures are below 60ºF. 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
See Figure 9
This unit is located at the right front fender inner panel on the Wagoneer and Cherokee, and on the front of the battery tray on the Commando. The high temperature vacuum signal modulator is connected to the EGR vacuum signal hose by a plastic T-fitting. The modulator opens when the underhood air temperatures reach 115ºF and it causes a weakened vacuum signal to the EGR valve, thus reducing the amount of exhaust gases being recirculated.