The ECU looks at the ECT (Engine Coolant Temperature) sensor to know when to actuate the EGR VSV. This enables the EGR valve to operate when the engine reaches normal operating temperature. No EGR flow is desired until the engine is warmed up.
After it's enabled, the EGR vacuum modulator controls when and how much the EGR valve is opened. No flow is desired at idle and under wide open throttle. Only intermediate throttle settings.
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