To send oil fumes and crankcase blow-by gasses back into the engine for burning, all engines are equipped with some type of crankcase breather control valve.
On early 8v engines, the crankcase ventilation system consists of a ventilation hose connecting a fitting on the top of the cylinder head cover to the airbox, and a secondary hose that attaching the intake manifold to the ventilation hose. A baffle under the hose fitting on the valve cover prevents excess oil vapor from making its way to the airbox. The small hose connecting to the intake manifold has a built-in restrictor orifice, which essentially creates a controlled vacuum leak to the intake manifold. When the throttle opening is small and manifold vacuum is high, crankcase oil fumes are drawn directly into the intake manifold. When the throttle opening is large and manifold vacuum low, some of the oil fumes flow through the large hose to the airbox. 16v engines use this same system, except the ventilation hose attaches to a plastic baffle on the cylinder block, instead of the cylinder head cover.
Later model 8v engines are equipped with a equipped with diaphragm control valves, which take place of the previously described system. The diaphragm control valve is mounted to the cylinder head cover, and controls ventilation of the crankcase and cylinder head into the intake tract. The diaphragm control valve maintains a constant balance against manifold vacuum, keeping blow-by vapor flow at a constant percentage of the total intake air volume.
VR6 engines use a simple crankcase ventilation system, similar to the 4-cylinder engines. A single hose connects the cylinder head cover to the intake tract, ahead of the throttle body. A heater element is contained within the hose, which prevents the vapors from icing over in cold weather. As with the 4-cylinder engines, the cylinder head cover contains a metal mesh flame trap that contains any ignited vapors during the event of an engine backfire.
On all systems, there is no maintenance required other than to check for vacuum leaks and clogged hoses. When removing hoses for inspection, take note of the direction of the restriction valves. Replace any clogged or cracked hoses.