Understanding EGR - How Exhaust Gas Recirculation Works
Emissions systems in today's vehicles are more complicated and effective than ever before, using a wide array of methods to reduce harmful emissions into our environment. One of the most common, and earliest developed of these components is the EGR (exhaust gas recirculation) system. The primary function of the EGR system is to recycle a portion of the exhaust back through the intake for re-combustion. The benefit of this recirculation of exhaust gasses is greatly reduced NOx (nitrogen oxide) emissions in both gas and diesel engines. In diesel engines, the EGR system further helps reduce soot particles created in the combustion process and as a side benefit reduces engine noise. On gasoline engines an added benefit, besides reduced emissions, is decreased fuel consumption.
The function of the EGR system is relatively simple, a portion of the exhaust gasses are siphoned off from the exhaust manifold and redirected into the fresh air intake stream. The key component is the EGR valve, which controls the rate at which the gases are redirected from exhaust to intake. There are two main types of EGR valves, earlier versions were pneumatically controlled through the vehicles vacuum system. While this gives some level of control the drawback is the system is influenced by the load on the engine and there are many points of failure (e.g. vacuum lines, pressure and check valves, etc.). The modern solution to this problem is updated electronics system tracking. Today EGR valves are controlled using solenoids or DC motors and are directed by feedback from numerous electronic sensors which feed to the ECU. The ECU in turn tells the EGR valve what position it should be based on a wide number of factors such as engine load, engine temperature and ambient air temperature, among others.
Sample Diagram of EGR:
As you may have guessed, the EGR valve lives its life in a brutal environment of high temperatures and repeated heat cycles. While modern EGR valves are pretty reliable they still are susceptible to failures. The most common failure is carbon accumulation in the valve body, which can cause the flow of gasses to be reduced, prevent the operation of the valve itself, or a combination of both. Other failure causes can be related to electrical faults, bad sensors or vacuum leaks in older systems. The first symptom most people notice when there is a fault in the EGR system is of course the check engine light on the dash. This indicates the OBD system has discovered an issue with some component in the engine. Other symptoms can include rough acceleration, overall poor engine performance and in diesel engines, an increase in black smoke from the tailpipe.
It is always recommended that you consult a workshop manual or trained technician familiar with your vehicle, but for reference, troubleshooting would typically involve using an engine scanning tool or computer to first pull the fault codes. A common code to see in a modern OBD II equipped car would be code P0401, which indicates insufficient EGR system flow. Once the problem is isolated to the EGR system various electrical tests should be carried out to verify the function of other components in the system. Depending on the results, you should replace any components that indicate they are faulty or test out of the parameters laid out by the manufacturer. If no faults are found during the electrical testing, a visual inspection of the EGR valve should be performed. This will usually indicate that the valve is jammed opened or closed because of carbon build up. If this is the case the valve will need to be replaced with a quality replacement part.