Crankcase Pressure
The pressure inside the engine crankcase caused by combustion blow-by gases.
Crankcase pressure is the air pressure built up inside the engine block's oil pan and crankcase area during engine operation. It is caused by blow-by—gases that escape past the piston rings during the combustion stroke. If not vented, this pressure can blow out oil seals and cause severe oil leaks.
To control this pressure, engines use a Positive Crankcase Ventilation (PCV) system. The PCV valve uses engine vacuum to draw these gases out of the crankcase and route them back into the intake manifold to be burned in the cylinders. This maintains a slight vacuum or neutral pressure inside the crankcase.
If the PCV system becomes clogged, crankcase pressure will rise, forcing engine oil past gaskets and seals (such as the oil pan gasket, valve cover gasket, or rear main seal). Excess pressure can also push oil into the air intake, fouling the intake system and spark plugs. Mechanics check for excessive pressure using a pressure gauge connected to the dipstick tube.