Here is a write up on what it does. A lot of people block them off. It may give you a small increase in power because of blocking extra heat from entering the intake. Cooler air is denser creating more power.
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.
