What Is an EGR Valve and What Does It Do?

EGR stands for exhaust gas recirculation, and the EGR valve is part of a system that does as the name implies: recirculate exhaust gas back into the engine’s intake manifold for a return trip to the cylinders it just left. That may seem counterproductive, rather like breathing in the same air you just expelled, but it’s only a relatively small amount of exhaust that’s recirculated, and it’s only done under certain operating conditions (which is where the “valve” part comes in). It also helps reduce exhaust emissions that cause air pollution.

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The air we breathe contains only about 20% oxygen, with almost all of the rest being nitrogen. Whether human or engine, the only part of air that does you any good is the oxygen; the nitrogen is inert, meaning it doesn’t do much besides take up space. However, if nitrogen gets really hot — as it can in an engine’s cylinders — it combines with some of the oxygen to produce nitrogen oxides (sometimes written as NOx and pronounced “knocks”) that contribute to pollution.

When exhaust is routed back into the cylinders, it takes up space that would otherwise be taken up by the combustible air-fuel mixture, so there’s less of a “bang” when the air-fuel mixture ignites. This creates less heat, which lowers cylinder temperatures and the amount of NOx produced.

How Does an EGR Valve Work?

The EGR valve is typically located somewhere along a pipe that runs from the exhaust manifold to the intake manifold. Older EGR valves are activated by engine vacuum through hoses, while newer ones are computer controlled. As mentioned, the EGR valve opens to allow exhaust through only under certain operating conditions. For instance, when you want maximum power under full throttle, the EGR valve is closed and doesn’t let any exhaust flow through — or at least that’s what’s supposed to happen. (Note that some newer cars with variable valve timing don’t have EGR valves. That’s because the “variable” part allows the exhaust valve to close earlier than it normally would under certain conditions, which traps some of the exhaust in the cylinder and therefore mimics what the EGR system does.)

Exhaust also contains carbon, and sometimes the carbon builds up in the EGR valve and makes it stick either open or closed, meaning it’s sometimes not in the right position. The EGR valve can also fail for other reasons: For instance, older versions that are operated by engine vacuum won’t work properly if the vacuum hoses leading to it leak or get clogged. Newer, computer-controlled EGR valves can do the same if the computer or motor that opens and closes the valve doesn’t work the way it’s supposed to.

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What Are the Symptoms of a Bad EGR Valve?

In most cases of EGR valve failure, the engine will do any or all of the following:

• Run roughly at idle
• Suffer hesitation, reduced performance or stalling
• Produce a knocking sound

A bad EGR valve should also cause the check-engine light to illuminate. If that’s the case, the best first step is to use a code reader or scan tool to read the computer-generated trouble codes, as several other problems can cause the same symptoms.

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