The EGR is an exhaust gas recirculation device used for the purpose of lowering emissions. The purpose of the EGR is to allow a certain amount of exhaust to return and be mixed with the incoming fuel into the cylinders at cruising speed. This helps to cool the cylinders, allowing for a higher degree of ignition advance to burn more of the fuel. This makes the engine more efficient. If the EGR is not used, the ignition timing would have to be retarded somewhat so that detonation would not occur.
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By increasing the ignition timing and allowing the fuel to have more time to burn, there is less unburned fuel reaching the catalytic converter, hence less emissions. The EGR cannot function at an idle, or it will cause a very rough running engine or stalling. It is meant to actuate at around a constant 2,000rpm. This is accomplished through an EGR vacuum control solenoid. The EGR control solenoid may be an on-and-off type switch, or it may be a modulated-pulse width type. Either type has battery plus on one side, and the computer operates the solenoid by grounding the other side when it wants to turn the EGR on.
The EGR has four wires, three of which are to the computer so it can not only turn the vacuum on to the EGR but also monitor its position. Some EGRs use a trio of monitors on the top of the EGR to monitor and to operate the EGR electronically without the use of the vacuum solenoid.
The EGR is prone to getting plugged up with carbon over a period of time and should be inspected if a rough idle is occurring. It can be blocked up or stuck open. Before it is removed to inspect and clean or replace, check that it is getting a vacuum (if it this type) and that the rubber diaphragm inside it is not leaking, causing a vacuum leak. Check the EGR by squeezing the diaphragm up with your fingers to see if it moves.