[longassname]

egr stands for exhaust gas recirculation. It does one thing in order to serve two purposes. It recirculates exhaust gas as the name suggests. It does this to reduce engine temperatures and improve fuel economy. Exhaust gas isn't combustible. The oxygen in it has already been used. So allthough it is relatively hot it's not as hot in the combustion process as fresh air is as it's oxygen burns with fuel so exhaust gas can be recirculated into the intake charge to reduce combustion temperatures; this can allow more ignition advance at light cruise which can improve fuel economy. Also, since exhaust gas is not combustible but takes up space in the cylinders it's like making the engine a smaller engine.

The important thing with egr is to hot have the valve open at the wrong time. If the valve is open at idle it is a relatively large passageway in comparison to the small air passageway provided to clean air through the idle air bypass valve and closed throttle body. Also if the valve is open during acceleration the engine will be ingesting uncombustible exhaust gas as part of the mix when it wants as much fresh air as possible to make as much power as possible.