Negative Back-Pressure EGR Valve
If there is little or no vacuum in the vacuum chamber of the EGR valve, the EGR valve will not open. When there is enough vacuum in the chamber, from the manifold vacuum port, the pintle will rise off of its seat and allow the EGR valve to open.
When the EGR valve opens, back-pressure in the hollow shaft decreases. As back-pressure decreases, vacuum opens the control valve and bleeds EGR control vacuum to the atmosphere, thus closing the EGR valve. This cycle occurs about 40 times every second under normal operating conditions.