Factory source 3512cf1 010 Pressure Regulating Valve - 860121882 Cylinder head coverD04-134-33+C – Shengsida

The first and most basic function is to cover and seal the cylinder head, keep the oil inside, and isolate contaminants such as dirt and moisture from the outside.  This may sound simple, but it is usually the most difficult aspect to do well.  The second function of the cylinder head cover is to isolate the oil from the air.  Oil mist will form during the operation of the engine.  The cooler inner surface of the cylinder head cover will accumulate oil mist, causing the oil to condense and flow back to the oil housing.  The cylinder head cover is also responsible for crankcase ventilation.  When the piston moves in the cylinder, pressure builds up inside the engine.  If ignored, this pressure will cause each seal to leak, resulting in reduced engine efficiency.