EGR Valve is part of the exhaust circulation system that reduces pollution-causing emissions.
Most older EGR systems use a vacuum regulated EGR valve while newer vehicles tend to have an electronic EGR valve to control exhaust gas recirculation.
At idle speed the EGR valve should look closed. There is no EGR flow into the manifold. The EGR valve remains closed until the engine is warm and is operating under load. As the load increases and combustion temperatures start to rise, the EGR valve opens and starts to leak exhaust back into the intake manifold. This has a quenching effect that lowers combustion temperatures and reduces the formation of NOx.
An EGR valve that is stuck open:
When the EGR is stuck open, it will lead to a vacuum leak which in turn causes inefficient combustion, hesitation, rough idling and even stalling.
An EGR valve that is stuck closed:
This causes nitrogen oxide emissions to rise and the car might start to knock.