Two Port System

Fig. 65 Single Port EGR System:

The EGR system recirculates a small amount of exhaust gas into the intake system to reduce the combustion temperature and reduce the production of NOx. Exhaust gas is led from the the exhaust port of the No. 2 cylinder through a port in the cylinder head and then through a steel line to the EGR valve. When the EGR valve opens, a small amount of exhaust gas passes through the port and into the the intake manifold. For an illustration of the two port EGR system, see Fig. 66.

Fig. 3 Two port EGR system:

The opening of the EGR valve is controlled by two vacuum sources on the throttle body. When the throttle blade is closed (idle) both ports are exposed to atmospheric pressure, A, Fig. 3. As the throttle blade opens, port 2 is exposed to manifold vacuum, B, Fig. 3. This vacuum is partially bled off through port 1 which is still exposed to atmospheric pressure. This reduces the vacuum signal to the EGR valve causing it to open less than it would otherwise. As the throttle blade opens further, port 1 is also exposed to manifold vacuum, C, Fig. 3. At this point the EGR valve opens completely. A thermostatic vacuum switch shuts off vacuum to the EGR valve at coolant temperatures below 112°F to improve cold driveability.