An Exhaust Gas Recirculation (EGR) valve is a mechanical component found on automobiles and plays an integral part in the proper functioning of gasoline and diesel-engined cars. It ensures that the car burns its fuel effectively by recirculating the emissions from the exhaust through the car’s combustion system. Properly recirculated fuel results in cooler and fuller burning, along with a reduction in the quantity of harmful gases, especially nitrogen oxide (NO). When NO meets the air, is turned into nitrogen dioxide, which in turn becomes smog when it encounters hydrocarbons. In vehicles where the EGR valve is faulty, the driver commonly finds that acceleration is difficult or that the car experiences rough idling or stalls.
The first EGR valve was installed in cars in 1972. They were originally mechanical, and have been superseded by the more modern and much more sophisticated electronic valves. The latest versions are not only capable of self monitoring, but they periodically create problems for the system to ensure the correct response is produced. On some systems, should system fail or encounter a problem, it will automatically produce a diagnostic trouble code (DTC).
The EGR valve is considered by the motor-manufacturing industry as a "metered intake leak" and was invented to lower the fuel combustion temperature. This component is involved in the combustion process for most of a car’s journey, save for when the car is at either end of its performance capacity: idle or full acceleration.
During full acceleration, the valve is disabled to encourage combustion and fill the combustion chambers, a set of conditions called wide open throttle (WOT). Were it to be invoked during full acceleration, it would inhibit the acceleration by reducing the capacity of the fuel combustion chambers. At the other end of the scale, when the car is idling, it is especially sensitive to the composition of air and fuel in the combustion chamber, and therefore the EGR valve is turned off. Should the component still be in operation when the car is idle, the vehicle is likely to stall or stumble.
Though not recommended, some vehicle owners remove or otherwise disable their EGR valves in the belief that it will improve the vehicles’ performance. A car with a disabled valve will not pass an emissions test, and this process is discouraged by the motoring industry.