EGR Valve

Exhaust gas recirculation dilutes the intake charge with inert gas. Combustion becomes slightly cooler, and less nitrogen oxide (NOx) forms. Diesels use high EGR rates, often with a cooler for the recirculated gas. The valve is opened by the control unit at part load and closed at idle and full power. Soot and oil vapour make deposits that can stick it open or shut.

Many diesels have an EGR cooler that lowers the temperature of the recirculated gas. Some engines use both high-pressure and low-pressure EGR circuits.

An electric or vacuum actuator opens the valve; a position sensor reports how far. The control unit checks the effect through air flow or pressure readings. The control unit opens the valve mainly at part load and moderate engine speeds. At idle on some engines and at full load, EGR is reduced or closed to keep the engine smooth and powerful.

A stuck EGR valve causes rough idle, stalling, black smoke or loss of power and codes such as P0401. Removing or blocking it is illegal in many countries and fails emission checks. Soot and oil vapour can clog the valve, especially on diesels used for short trips. Symptoms include rough running, low power, black smoke and codes such as P0401 or P0400.