Exhaust Gas Recirculation Valves Explained

Under certain temperature conditions inside a running engine, nitrogen and oxygen combine to form sky-darkening nasties known as nitrogen oxides, or NOx. These compounds combine with other tailpipe crud known as hydrocarbons and, presto, smog—the kind of air that can be seen. One way to help an engine reduce these smog-forming gases is to recirculate spent exhaust back into the engine by way of the Exhaust Gas Recirculation (EGR) system.

Function
At the heart of this system is a valve that meters the exhaust back into the engine. The working concept behind an EGR system is simple. A small amount of exhaust gas is recirculated back into the combustion chamber when the engine is prone to producing nitrogen oxides. Since exhaust gas is inert, in the sense that it cannot be burned again, combustion temperatures are reduced and, thus, the production of NOx is lessened. By way of a series of valves, passages, ports, relays, solenoids, and even computers, the exhaust gas is reintroduced on the intake side of the engine. Combustion temperatures are cooled, and nasty NOx production is kept at bay. In concept, the EGR system is simple, yet in practice things can become rather complicated.


When the EGR system is working properly, it performs its job without complaint or problem. When one or all of the components fail, too much or too little exhaust gas can be reintroduced into the combustion chamber at the wrong time and wreak havoc on smooth engine operation. Exhaust gas reintroduced at idle can cause rough idling and stalling due to the unwelcome addition of carbon dioxide to the already fragile balance of fuel and air.

Failure
If the EGR fails at full-throttle, combustion temperatures can become too high causing knocking, or pinging, otherwise known as detonation. This sounds like a steel coffee can full of large ball bearings under the hood. What makes this sound is the explosion of fuel before the piston reaches the top of its compression stroke. These too early explosions cause tremendous stress on engine components and can bring an early demise to an otherwise healthy powerplant. While a stalling or rough-running engine can be an annoyance, a detonating engine can be downright expensive. A malfunctioning EGR system that malfunctions in conjunction with computerized management can also trigger the dreaded CEL, or check engine light.


The usual culprit in the case of a malfunctioning EGR valve is the valve mechanism itself getting stuck open, closed, or somewhere in between. The good news is that EGR valves are fairly easy to replace. The bad news is that the valve itself may not be the reason for its own failure to cooperate. Depending on the complexity of the EGR system, the cause of a malfunctioning EGR valve may be as simple as a loose or cracked vacuum line to as complex as a fussy and hard-to-isolate electronic sensor.