The Positive Crankcase Ventilation (PCV) system leads blow-by gases and crankcase vapors into the intake manifold allowing them to mix with intake air then to be sucked into the combustion chamber and burned together air/fuel mixture. This system fresh air from the air filter housing is channeled through hoses into the crankcase, where the fresh air gets to mix with the blow-by gases before the intake vacuum sucks it through the PCV valve and into the intake manifold. High intake manifold vacuum under idle and part-throttle helps draw blow-by gases and crankcase emulsion vapor via the PCV valve and through the crankcase ventilation hose into the intake manifold while at the same time drawing fresh air through the air intake duct or air filter housing into the crankcase. While, there is no specific timing for checking free the PCV valve or the hoses, the system slowly becomes less efficient because of oily sludge deposits and crankcase pressure build-up, which leading to symptoms like leaking engine seals. PCV hoses should be checked during such maintenance activities as the change of the oil filter, air filter, fuel filter, or spark plug; where dirty, the hoses should be washed; where cracked or deteriorated, the hoses should be renewed. On all models, the PCV valve is fastened to the valve cover; it should also be checked and replaced, when needed.