What does the EGR system actually do on a diesel engine? In modern inline-six engines used in semi trucks, exhaust gases don’t just leave the engine — a portion is routed back into the intake through the EGR system.As exhaust gases drive the turbo and fresh air is cooled and fed into the intake, the EGR valve diverts a controlled amount of exhaust through a cooler and reintroduces it into the intake stream. This process is called Exhaust Gas Recirculation (EGR).EGR isn’t designed for performance