2005 Mercury Mountaineer PCV Valve Replacement Cost

As the engine operates, small fires inside each engine cylinder create pressure that pushes the engine piston down, toward the crankshaft, and some combustion gases from the fire in the cylinder are able to get around the piston, and enter the crankcase. Also, heat from the engine causes the air inside the crankcase to expand. If this condition were left untreated, the engine would develop leaks, and would suffer severe inefficiency, if it ran at all. To prevent this situation, the crankcase ventilation system connects the crankcase to the intake manifold. The intake manifold is constantly under vacuum as the engine sucks in air, so when the crankcase ventilation valve is installed, the hot gases are sucked from the engine crankcase, and burned in the engine as fuel.

Failed PCV valves can cause the engine to idle roughly and the Check Engine Light to illuminate.