Exhaust Gas Recirculation: Description and Operation

Fig. 50 EGR System Wiring And Vacuum Hose Diagram:

The EGR system is designed to control the emission of oxides of nitrogen (NOx) by recirculating a part of the exhaust gas into the intake manifold through the EGR valve.
The EGR valve is operated by ported vacuum and provides EGR proportional to engine load by the operation of EGR control valves "A" and B.
EGR control valves "A" and "B", which control the EGR flow rate, operate as follows:

1. Ported vacuum is directed to the diaphragm for EGR Control valve "A" and the valve is opened.
2. When EGR Control valve "A" opens, manifold vacuum is bled into the air filter through an orifice in the air passage. This generates vacuum across the orifice in the air passage and this vacuum is applied on the upper diaphragm of EGR control valve "B", so that EGR valve "B" begins to open.
3. As EGR control valve "B" opens, ported vacuum is bled off to the carburetor venturi, and this makes composite vacuum (Pe) which is applied on the diaphragm of EGR valve "A". As Pe vacuum decreases, EGR control valve "A" begins to close and the vacuum in the air passage decreases. This makes EGR control valve "B" begin to close.
4. As EGR control valve "B" closes, composite (Pe) vacuum increases, EGR control valve "A" begins to open and the steps from 1 to 4 are repeated.

Because these steps are repeated very rapidly, carburetor intake air volume (flowing through the venturi) and the air volume through the air passage have a constant ratio, proportional to the carburetor intake air flow rate, and therefore to engine load.
The EGR system has an EGR cut-off function in response to engine coolant temperature and vehicle speed. When the engine is cold, combustion gas temperature is low and the engine emits relatively low NOx emissions. Therefore EGR flow is cut off by the purge cut-off solenoid valve in order to assure good cold engine driveability. EGR flow is also cut whenever the vehicle is not moving.