The valve that controls exhaust gas recirculation in diesel engines is the EGR valve.
The EGR valve meters how much exhaust gas moves from the exhaust side of the engine into the intake. Depending on the design, it may be operated by a vacuum actuator, an electric motor, or an electronically controlled pneumatic system. The engine-control unit opens and closes it—or positions it continuously—to balance emissions reduction with drivability, power, and fuel economy.
When the valve opens, exhaust dilution lowers the oxygen concentration and peak combustion temperature, helping reduce NOx formation. When the engine needs full power, is still warming up, or is operating under conditions where recirculation would be unhelpful, the valve may close or reduce flow. The exact strategy varies by engine generation and emissions standard.
The EGR valve is commonly confused with the EGR cooler, but they perform different jobs. The cooler lowers exhaust temperature; the valve controls flow. An EGR filter is not the normal name for the metering component, and an EGR pump is not a standard substitute for it. Soot and oil vapour can build up around the valve and intake passages, causing sticking, restricted flow, poor acceleration, or an illuminated engine warning lamp. Diagnosis requires checking commanded position, actual flow, sensors, and related passages rather than replacing the valve automatically.