The basics of EGRs - what they do, how they work, how to troubleshoot

Integral to the vehicle’s engine management system, the aptly named exhaust gas recirculation valve, or EGR valve for short, recirculates finely metered quantities of exhaust gas to the engine intake system for increased engine efficiency, reduced fuel consumption and lower NOx emissions.

With growing pressures to reduce emissions, the EGR valve will play an increasingly important role moving forward. It's important to know what it does, why it fails and how to replace it when it does.

How does an EGR valve work?

Nearly 80 percent of the air we breathe is nitrogen. However, when it is exposed to the extremely high temperatures in the combustion chamber, plus 1370°C, the normally inert gas becomes reactive, creating harmful oxides of nitrogen or NOx, which are then passed through the exhaust system into the atmosphere.

To help minimize this, the EGR valve allows a precise quantity of exhaust gas to re-enter the intake the system, effectively changing the chemical makeup of the air entering the engine. With less oxygen, the now diluted mixture burns slower, lowering temperatures in the combustion chamber by almost 150°C, and reducing NOx production for a cleaner, more efficient exhaust. 

The EGR valve has two primary settings: open and closed, although the position can vary anywhere in between. The EGR valve is closed when the engine is starting up. During idle and at low speeds, only a small amount of power is required, and therefore only a small amount of oxygen, so the valve gradually opens – it can be up to 90% open at idle. However as more torque and power is required, for example during full acceleration, the EGR valve closes to ensure as much oxygen enters the cylinder.

As well are reducing NOx, EGR valves can be used in downsized GDi engines to reduce pumping losses and improve both combustion efficiency and knock tolerance. In diesel, it can also help to reduce diesel knock at idle.