Exhaust Gas Recirculation: Description and Operation

Fig. 3 Exhaust Gas Recirculation (EGR) system:

Fig. 4 Positive backpressure type EGR valve:

Fig. 5 Negative backpressure type EGR valve:

Fig. 6 Ported vacuum type EGR valve:

Fig. 7 EGR valve identification:

The EGR system, Fig. 3, lowers temperature within the combustion chambers to reduce nitrogen oxide emissions. The main element of this system is the EGR valve which is operated by vacuum and mounted on the intake manifold. This valve feeds small amounts of exhaust gas back into the combustion chamber.
The EGR valve is mounted on the intake manifold and controls exhaust gas flow into the intake manifold. The exhaust gas then moves with air/fuel mixture into the combustion chamber. Only a small portion of the exhaust gas is allowed to pass through the valve when the motor is idling. The EGR valve is usually open when operating a warm engine at idle speed.
There are three types of EGR valves. The positive backpressure type, Fig. 4, has an air bleed, located inside the EGR valve that acts as a vacuum regulator. This valve controls the amount of vacuum in the chamber by bleeding during the open phase of the cycle. When the valve receives exhaust backpressure through its shaft, it closes the bleed. At this time maximum vacuum is applied to the diaphragm and the EGR valve opens. If there is little or no vacuum or no pressure in the exhaust manifold, the EGR valve will not open. The negative backpressure EGR valve, Fig. 5, is very similar to the positive backpressure valve except the bleed valve spring is located below the valve and the valve is normally closed. The amount of exhaust gas will always be different into the manifold depending on vacuum and variations in exhaust backpressure. The ported EGR valve, Fig. 6, uses vacuum connected direct to the EGR valve. This amount is controlled by the amount of manifold vacuum and the throttle opening. All three valves have identification stamped after the part number, Fig. 7.