EGR System: Operation
Exhaust backpressure operates the EGR modulator diaphragm which operates the EGR valve. Under low load, exhaust pressure is low and a spring pushes the EGR diaphragm down. The modulator valve opens to let outside air into the vacuum passage, lowering vacuum and slightly opening the EGR valve.
Under high load, the modulator diaphragm is pushed up and the modulator valve closes. Air does not enter the vacuum passage, so vacuum to the EGR valve is larger and the EGR valve opens wider.
When coolant temperature is low, the Bi-Metallic Vacuum Switching Valve (BVSV) opens the EGR valve vacuum passage to outside air and the EGR valve remains closed.