On a gasoline engine, when the temperature of the combustion chamber approaches 2500 degrees Fahrenheit, nitrous oxide emissions increase. The fuel air mixture can also spontaneously combust, causing valve clatter and damaging the engine. Allowing a small amount of exhaust gas back into the intake reduces combustion chamber temperature. Regulating the flow of exhaust gas is the job of the EGR system.
EGR stands for exhaust gas recycle
The most basic form of EGR would be a hole between the exhaust and intake manifolds. In the early 1970's EGR systems were little more. It was soon discovered that too much EGR or applying it at the wrong time creates problems. Over the years hundreds of EGR designs have been developed. Every design attempts to serve the same purpose, regulating exhaust flow.
Most early EGR valves were vacuum-operated. A vacuum diaphragm opened and closed a valve, allowing and cutting off exhaust flow. An early refinement was a temperature-controlled shut-off in the vacuum source. This kept the EGR valve from opening when the engine was too cool. The cool engine did not require EGR and cutting it off made the engine run smoother.
EGR flow is also undesirable at other times, for instance at idle. At very low speed, combustion temperature is naturally lower. Adding exhaust gas at low speed can cause rough idle. The positive back-pressure EGR valve helped solve this problem.
Similar to a standard vacuum model, the positive back-pressure design has a hollow valve stem. This allows exhaust gas pressure to push against a spring loaded vacuum valve. When back pressure rises, such as on acceleration, exhaust pressure closes the spring-valve and seals the vacuum opening. This allows an engine vacuum to open the EGR valve.
When back pressure is low, such as at an idle, the spring opens the vacuum port. Engine-vacuum is bled off and the EGR valve closes. The design change has caused many good EGR valves to be replaced needlessly.
Vacuum operated valves are often tested by applying vacuum to see if the valve opens. If the valve does not open or if the vacuum cannot be held, the valve is considered bad. This works okay on a standard or on a negative back-pressure EGR valve.
A positive back-pressure valve will not hold a vacuum or operate without positive back-pressure. Testing this valve by that method will give false results. The positive back-pressure valve would have to be tested with the engine speed raised to around two-thousand RPM, creating positive back-pressure.
EGR valves on modern vehicles are controlled by the engine computer as an output, using several sensors as inputs. Ford uses a differential pressure feedback EGR or DPFE sensor. This monitors exhaust back pressure before and after the valve. By comparing the two, EGR flow can be more precisely calculated. The vacuum source is regulated to open and close the EGR valve and control exhaust flow.
Electrically operated EGR valves are also common. These may have up to three valves in one unit. The engine computer can open and close any or all the valves in response to input from sensors. For instance the electronic temperature control or ETC sensor informs the computer when the engine is cold. Knock sensor(s) signal valve clatter or detonation. A sensor built into the unit informs the computer of the position of each valve.
Problems with EGR
Because exhaust gas contains carbon, the passages though which it flows are prone to stopping up. When this happens, the flow is interrupted. On post 1996 vehicles this will set a diagnostic trouble code or DTC and the check engine light will come on. These codes are very often misdiagnosed as a bad EGR valve.
Many engines have multiple EGR ports for discharging exhaust into the intake. If several ports plug up, all flow can be directed to a single cylinder. This can cause a misfire, which is very often misdiagnosed. Countless injection flushes and "tune ups" are needlessly sold for this problem, without success. Removing the intake and clearing the passages are the only repair.
The carbon in the exhaust gas can also coat the throttle body plate. When this happens, rough idle and dying can occur. The engine may set a general misfire code, which is often misdiagnosed. Cleaning the throttle body will generally clear the problem.
Actual failure of the EGR valve is rare
An EGR valve can stick open when it should not, causing a rough idle or hesitation. It can also stick closed, causing valve clatter and a check engine light. Far more common are failures of the sensors that read and monitor the flow or the hoses and wires that control the system.
An EGR code needs to be properly diagnosed to learn the cause. Replacing parts without understanding how the system operates, can be a very expensive lesson in Swaptronics.