Part of the positive crankcase ventilation system, which reroutes crankcase blow-by to the intake manifold and back to the engine, where it's reburned in the cylinders as part of the fuel/air mixture. This cuts emission pollution and increases fuel economy because unburned fuel in the blow-by is consumed the second time around. It also keeps the blow-by and water vapor from fouling the oil in the crankcase, thus reducing the formation of engine sludge.