EGR valve the top of box on top of the inlet manifold of a Saab H engine in a 1987 Saab 90. Within modern, diesel internal combustion engines, Exhaust Gas Recirculation ( EGR) is a method to control Nitrogen Oxides (NOx) emissions,. The EGR valve is fairly simple when it is open exhaust gases are allowed through the EGR system to help control vehicle emissions, whereas. In this episode of Driveway Dudes, Paul and Will shows you how to clean your EGR (Exhaust. Exhaust gas recirculation (EGR) is an emission control technology allowing significant NOx emission reductions from most types of diesel engines: from light-duty engines through medium- and heavy-duty engine applications right up to low-speed, two-stroke marine engines.