Exhaust Gas Recirculation (EGR) Valve
WARNING: This page is about a different car, the 1995 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.
The EGR valve recirculates a portion of the exhaust gas back into the engine to lower combustion temperature and reduce the amount of oxides of nitrogen created during combustion. The amount of exhaust gases that are released into the engine are proportional to engine load.