EGR Valve: Description and Operation

At idle both vacuum ports are on the upstream side of the throttle butterfly, no vacuum or pressure can act upon the EGR diaphragm and the EGR valve remains closed.

Fig. 67 Vacuum Schematic:

When the throttle butterfly is partially open and the turbocharger is operating, the pressure upstream of the butterfly valve will be higher than the pressure downstream of it. This pressure differential will hold the EGR valve open, despite the vacuum signal received form port A, Fig. 67.
When the throttle valve is slightly open, port A, Fig. 67, on the throttle valve housing will be downstream of the throttle valve and will therefor apply a vacuum signal to the signal converter. The EGR will then allow a small amount of exhaust gas onto the intake system.
When the throttle valve is open, the pressure signal from both ports will be about equal, and no EGR flow will result.