The applied vacuum pulls against a diaphram in the EGR valve and mechanically pulls a pintle out of its seat allowing EGR gasses to flow through the valve. When vacuum is released, the spring-loaded pintle returns to its seat and seals off the flow of EGR gasses.
EGR flow should occur only under the control of the PCM when driving conditions are such that EGR flow will lower combustion temps and keep NOx emissions down. The PCM does this by applying variable duty cycle power to the EVR (EGR Vacuum Regulator) solenoid. Depending on the duty cycle, a varying amount of vacuum will be switched through by the EVR solenoid to the EGR valve allowing control of the amount of EGR flow.
The computer uses the DPFE sensor to measure the amount of EGR flow by measuring the difference in pressure across an orifice that is exposed to the EGR flow. The greater the rate of flow, the greater the difference between the two ports of the sensor. The computer uses this feedback to achieve the desired amount of EGR flow.
If the ports in the throttle body clog (most common reason), the EGR valve will be opened fully but the DPFE sensor cannot sense any or sufficient flow. The PCM sees this and sets the P0401 fault code.
The other common failure is the DPFE sensor. It cannot measure the actual flow and sets the same code.
The purpose of applying vacuum to the EGR valve during idle is to see if the EGR valve will open and allow EGR gasses to pass all the way to the intake. This is indicated by the stalling or stumbling when the EGR valve is opened on a properly functioning system.
Steve
