Valvoline.com

THE ABCs of EGR, Part I

Learning the value of the valve

Unless your vehicle fails an exhaust emissions test due to a high NOx reading, you probably haven't needed to know much about your vehicle's EGR (Exhaust Gas Recirculation) system. That's fine—you have enough to worry about already, right? Still, it's a vital component so it's important to understand how it works, which might help when making repairs to restore its function.

In this first of a 2-part series, we'll provide some background and overview of the EGR system, including the reason for its existence. And in our second part, we'll dig into some specifics on maintenance and repair.

Purpose

The EGR system made its debut on pollution-controlled motor vehicles in the U.S. in the early 1970s, preceding the catalytic converter by a couple of years. Its sole function was to reduce NOx emissions by recirculating exhaust gas into the engine's intake system during above-idle driving conditions.

NOx is often referred to as nitrous oxide, but correctly known as oxides of nitrogen when produced as an exhaust byproduct of the internal combustion process. The higher the combustion temperatures go, the more NOx emissions are produced.

Other factors that affect NOx production include combustion pressure (compression ratio), ignition timing, fuel mixture, intake air temperature, and coolant temperature. Of course, variations in these conditions also affect other exhaust by-products, such as carbon monoxide and hydrocarbon emissions, as well as overall engine performance and fuel economy. Balancing these factors to achieve optimal results in the areas of engine performance, emissions control, and fuel economy has been an engineering challenge, to say the least.

Balance

For example, lowering the compression ratio and retarding ignition timing on a high-performance engine will lower NOx emissions, but also reduce peak horsepower and engine performance. That actually happened to the vaunted powerplants propelling the famous musclecars of the late Sixties and by 1972 they had considerably less muscle.

On the other hand, while leaning-out the fuel mixture and raising the coolant temperature will lower carbon monoxide emissions and improve fuel economy on most engines, driveability may suffer and higher levels of NOx emissions will be produced. The "fuel crisis" in the mid-Seventies placed added importance on the issues of emissions and economy.

Control

The EGR system became the front running candidate for the control of NOx emissions, due to its flexibility in applications and low cost. Early examples of this system were basically just added on to the engine designs existing at the time. With a few exemptions, notably the Honda CVCC design, almost all engines in the later Seventies used this system. However, while NOx emissions were reduced, the vehicle's driveability often suffered. Disconnecting the system and driving the vehicle without it easily confirmed this fact.

As time passed and engineers integrated the EGR system into upgraded or newer engine designs, this drawback was eliminated. Catalytic converter technology improved as well, as new designs now reduced NOx emissions (earlier designs only lowered hydrocarbon and carbon monoxide emissions). As computer-controlled fuel and ignition systems appeared and improved, disconnecting the system actually had an opposite effect on driveability, in contrast to early EGR systems.

On the other hand, on very efficient engine designs with hyper-accurate fuel and ignition management systems, an EGR system was often not even necessary! If your vehicle's engine is one of these, then reducing its NOx emissions will involve other repairs not covered in the following article. But if your vehicle does have this system, which still appears on some new models, then a non-functional EGR system is probably the culprit. This would be especially true if the vehicle is more than five years old and/or has more than 75,000 miles on it.

All EGR-equipped vehicles have at least an EGR Valve, an exhaust passage into it, and an intake passage from it. The valve is controlled either electronically or by vacuum, so that it opens at the appropriate time, allowing exhaust gas to enter the intake system.

If your vehicle has an electronically controlled valve, testing and servicing it will not be covered in this article (that requires a pro with some specialized diagnostic equipment). For those with a valve controlled by vacuum—and many are—then check out the next installment. Consult a service manual if you aren't sure about which type of control the system on your car uses. We'll see you in Part Two.