When combustion temperatures exceed 2500 degree F., atmospheric nitrogen begins to react with oxygen during combustion. The result is various compounds called nitrogen oxides (NOX), which play a major role in urban air pollution. To reduce the formation of NOX, combustion temperatures must be kept below the NOX threshold. This is done by recirculating a small amount of exhaust through the "exhaust gas recirculation," or EGR. valve.
The EGR valve controls a small passageway between the intake and exhaust manifolds. When the valve opens, intake vacuum draws exhaust through the valve. This dilutes the incoming air/fuel mixture and has a quenching effect on combustion temperatures which keeps NOX within acceptable limits. As an added benefit, it also reduces the engine's octane requirements which lessens the danger of detonation (spark knock).
The EGR valve consists of a poppet valve and a vacuum diaphragm. When vacuum is applied to the EGR valve diaphragm, it pulls the valve open allowing exhaust to pass from the exhaust manifold into the intake manifold. Some engines have "positive backpressure" EGR valves, while others have "negative backpressure" EGR valves. Both types contain a second diaphragm that modulates the action of the valve. This prevents the valve from opening unless there is a certain level of exhaust backpressure in the system. EGR valves are calibrated for specific engine applications. The wrong valve may flow too much or not enough exhaust and cause emission, driveability and detonation problems.
EGR valves do not normally require maintenance or replacement for preventative maintenance. But the valve can become clogged with carbon deposits that cause it to stick or prevent it from closing properly. Dirty EGR valves can sometimes be cleaned, but replacement is necessary if the valve is defective.
Some newer engines are so clean from a NOX emissions standpoint that no EGR valve is required.
Another component that may be a part of your vehicle's EGR system is a "ported vacuum switch" (PVS), which may also be called a "thermal vacuum valve" or "temperature vacuum valve" (TVS). The switch controls the passage of vacuum that operates the EGR valve.
This device is a heat-sensitive switch that remains closed until the engine's coolant reaches a certain temperature. The PVS screws into the intake manifold, thermostat housing or engine so the heat-sensing element is in contact with the engine's coolant. Inside the switch is a wax plug that pushes a sliding plunger to uncover or block vacuum ports in the switch. As the engine heats up, the wax expands and pushes the plunger up until it uncovers or blocks the vacuum port. At this point, vacuum to the device that the switch controls is either applied or blocked. Severe engine overheating can damage the switch, making replacement necessary.
A solenoid is another device that is often used to control vacuum to the EGR valve. A solenoid is a magnetic coil attached to a plunger that uncovers or blocks a vacuum port. The solenoid vacuum port may be normally open or closed depending on the application. When voltage is applied to the solenoid, the coil moves the plunger which either opens or blocks the vacuum port. Voltage to the solenoid may be routed through a relay or timer and is usually controlled by the engine computer on newer cars.
When used to control the vacuum to an EGR valve on a late model engine, the engine computer will wait to energize the solenoid until the engine is warm and is operating above a certain rpm.