Exhaust Gas RECIRCULATION: Operation

On dual diaphragm EGR valves, vacuum signal is supplied by a timed vacuum port in carburetor and direct intake manifold vacuum. Amount of exhaust gases introduced into manifold can be controlled by throttle opening plus engine load. Upper diaphragm receives carburetor ported vacuum and lower diaphragm receives vacuum directly from intake manifold. Lower vacuum chamber controls opening of EGR valve with respect to engine load. During high manifold vacuum (part throttle cruising), EGR valve is open just enough to allow correct amount of exhaust gases into intake manifold air/fuel mixture. Forces tending to open and close valve are balanced. Then manifold vacuum drops, (during acceleration), vacuum between two diaphragms drops, allowing upper diaphragm to open further. This increases amount of exhaust gases introduced into intake manifold. Single diaphragm EGR valve works in a similar manner except it receives vacuum from carburetor port only, and is not as sensitive to engine load conditions. Purpose of Thermal Vacuum Switch, (located in thermostat water outlet), is to cut off vacuum to EGR valve until water temperature reaches 100°F.