System Description
Fig. 12 EGR valve & position sensor:
Fig. 13 EGR system schematic:
This system consists of an EGR valve/sensor assembly, Figs. 12 and 13, dual EGR control solenoids, and the EGR cooler assembly. The EGR gas flow is controlled by the ECA, which uses engine manifold vacuum to operate the EGR valve.
The EGR valve is attached to the intake manifold under the carburetor. The valve controls the flow of gases through a tapered pintle valve and seat. A position sensor attached to the valve provides an electrical signal to the ECA which indicates EGR valve position.
The EGR valve used in this system is completely covered, and no pintle movement can be seen when the valve is installed. The valve and position sensor are serviced as individual units.
The EGR valve flow rate is controlled by dual EGR control solenoids mounted on a bracket attached to the left hand valve cover, Fig. 13. Proper control of the vacuum used to operate the EGR valve requires two types of solenoid valves: a vent valve which is normally open, in which the outlet port is normally connected to the inlet port when the solenoid is not operated; a vacuum valve which is normally open, in which the outlet port is normally blocked when the solenoid is not operated.
In order to provide improved flow characteristics, better engine operation and EGR valve durability, an external EGR gas cooler, Fig. 13, is used to reduce exhaust gas temperature. The cooler assembly is mounted over the right valve cover and uses engine coolant to reduce the temperature of exhaust gases routed from the exhaust manifold to the EGR valve.
During operation, the EGR valve is operated by vacuum supplied from the intake manifold. The vacuum and vent solenoids function together, as determined by the ECA to:
a. Increase EGR flow by applying vacuum to the EGR valve.
b. Maintain EGR flow by trapping vacuum in the system.
c. Decrease EGR flow by venting the system to atmosphere.
With information received from the various sensors, the ECA determines the correct amount of EGR flow required, checks the position of the EGR valve pintle, and determines if a change in position is required. In response to these calculations, the ECA puts the EGR system into one of the modes mentioned previously. The ECA samples and calculates these changes about ten times each second, for improved fuel economy and driveability under all conditions.
Fig. 39 - EGR Valves:
There are three types of EGR valves, the poppet type, modulating type and the tapered stem type, Fig. 39.
The poppet type valve consists of a spring loaded diaphragm, and a valve stem and valve operating in an enclosed valve body. At approximately 3 inch Hg of vacuum, the valve begins to open. The valve stem is pulled forward unseating the valve and allowing exhaust gas to flow into the valve chamber. Venturi vacuum will then pull the gas from the chamber into the air-fuel flow and then into the combustion chambers. Once the valve has been unseated, the only means of limiting exhaust gas flow is the size of the flow restrictor placed in the inlet port of the valve body. The size of the restrictor will vary according to engine application.
On the modulating type valve, an additional disc has been added to the valve stem below the main valve. The modulating valve operates exactly like the poppet valve when vacuum is between approximately 3 in Hg and 10.5 in Hg. When vacuum reaches approximately 10.5 inches Hg., the lower disc (high vacuum flow restrictor) approaches the shoulders of the valve seat and restricts the flow of exhaust gas. The purpose of the modulation of gas flow is to improve driveability on certain engine models.
The tapered type valve resembles the poppet and modulator types in function, except that the stem portion protrudes below the mounting flange.
NOTE: The EGR valve and vacuum control valve cannot be repaired and must be replaced if damaged.
Fig.20 - Dual Area Diaphragm:
Dual Area Diaphragms - On 1973-80 vehicles, dual-area diaphragms are used, Fig. 20. These diaphragms offset effects of engines using the EGR system and equipped with automatic transmissions. The new diaphragms permit vehicles to function with satisfactory shift spacing and shift feel.
To test, remove the vacuum diaphragm and test unit using an outside vacuum source. Set regulator on tester to 18 in. Hg with end of vacuum hose blocked off then connect vacuum hose to vacuum diaphragm unit. If unit does not hold 18 in. Hg reading, the diaphragm is leaking and must be replaced.