EGR System: Operation
Thermo valve is connected to radiator. Below 59°F (15°C) it is closed. Above 59°F (15°C), thermo valve opens vacuum circuit from intake manifold to differential pressure switch and to vacuum reservoir.
EGR valve opens or closes passage from exhaust manifold to intake manifold. Amount of exhaust gas allowed to pass is controlled by orifice located in exhaust manifold. Vacuum causes EGR diaphragm to open valve. Source of vacuum is close to throttle.
Vacuum is controlled by EGR valve solenoid. Solenoid has voltage applied at all times. Differential pressure switch controls ground circuit for solenoid. Differential pressure switch has 2 chambers separated by a diaphragm. The diaphragm has a calibrated hole. Diaphragm either opens or closes internal ground circuit contact.
The chambers of the differential pressure switch are connected to intake manifold via the thermo valve on one side and the other via the vacuum reservoir. When engine is operating at a stabilized RPM, pressure is equal on both sides of diaphragm, ground circuit contact is open, solenoid is de-energized and EGR valve closes.
If throttle is opened the vacuum in one chamber decreases immediately. The vacuum differential between the 2 chambers causes diaphragm to move and close ground circuit contact. Solenoid is then energized.
Vacuum is applied to EGR valve diaphragm and exhaust gas flows to intake manifold. Because vacuum decreases, spring forces diaphragm and opens ground circuit contact. Solenoid is de-energized and EGR valve closes.