The Exhaust Gas Recirculation (EGR) system helps to reduce vehicle emissions by introducing a small amount of exhaust gas into the engine's intake manifold. Re-burning this used fuel reduces combustion chamber temperatures, which play a huge roll in the formation of dangerous emissions like oxides of nitrogen. EGR failures can be subtle and are usually progressive.
An EGR system starts at the exhaust valve or manifold, where a small channel redirects a certain amount of the spent gases toward the EGR valve. Then, depending on the type of valve, it will be opened by the engine computer, engine vacuum or pressure of the exhaust. Once the valve is open, a predetermined amount of exhaust flows into the intake manifold, and from there into the combustion chamber.
Essentially, there are only two types of failure: the engine receives either too much or too little exhaust gas. Failures are usually due to the EGR valve sticking open or shut, but can also be due to a leak in the system. The EGR valve can stick because it is not receiving the vacuum or electric signal to move, or because it is dirty and encrusted with carbon.
An inoperative or clogged EGR valve will result in a rough idle, poor fuel economy and possibly a loss of power. Loss of power is especially prevalent with valves that are stuck open, as this will permit hot exhaust gases to constantly enter the combustion chamber when they are not needed. These gases will displace the air and fuel that your engine needs, causing a loss of power.
Electronically controlled EGR valves have a position sensor that tells the computer how far open or shut it is. If the computer does not receive the feedback it anticipates, it will trip a check-engine light. A valve that is stuck open will usually cause the check-engine light to come on after the car warms up. One that is stuck shut will trigger when the engine is cold, but might disappear when it warms up.
An EGR is primarily designed to reduce the formation of the toxic pollutant nitrogen oxide. One sure sign of EGR failure is a spike in the production of this chemical, which can only be detected by an emissions tester.
Additionally, a vehicle's computer will attempt to compensate for high combustion chamber temperatures by injecting extra fuel or retarding the ignition timing. This approach produces excess carbon monoxide and allows unburnt hydrocarbons to exit the exhaust stream. The latter can be recognised by a strong odour of gasoline from the tailpipe, but the former can only be detected by emissions testing.