An exhaust gas recirculation (EGR) valve is responsible for recirculating a small amount of exhaust into the combustion chamber. This may seem illogical, as exhaust gases need to be removed for the next combustion to take place properly. However, recirculation of these gases actually helps to cool down the combustion chamber and reduce emissions.

Specifically, reintroducing exhaust gases to the combustion chamber stops the formation of nitrogen oxides, which occurs when temperatures get too high in the chamber. The presence of the gases during combustion dilutes the air-to-fuel mixture just enough to keep the temperature of the combustion low.

EGR valves have been around since the 1960s, designed to improve engine efficiency and cut down on the amount of noxious gases created by the engine. The earliest models often caused difficulty starting, rough idling, a loss of performance and a decline in fuel economy. In 1973, Chrysler introduced a few different methods of regulating the EGR system, which were put in place to prevent the exhaust system from extracting all of the gases until the engine temperatures had reached the appropriate levels. This was one of the first steps in improving the design of EGRs.

EGR valves are either mechanical or controlled electronically, but the former is far more common than the latter. In a mechanical setup, the valve opens to allow gases back into the combustion chamber. Some setups rely on back pressure to open the valve, which prevents it from opening when unnecessary, such as when the engine is first started and needs the heat to warm up faster. Electronic EGRs work the same way, but rely on electronics to control the valve.

As time goes on, the EGR can become clogged with bits of debris or carbon deposits. The valve may also become stuck in the open or closed position. When the valve cannot function properly, the engine will either overheat or become overwhelmed with exhaust gases and thus unable to properly burn gasoline. This may result in rough idling, difficulty starting the vehicle, poor fuel economy and a decline in performance. Replacing the valve is typically recommended when it becomes clogged, but in some cases it may be able to be cleaned.