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Home > Video > Video Display > Engine Crankcase Pressure and Engine Oil Leaks

Engine Crankcase Pressure and Engine Oil Leaks

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**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**

An abnormally high crankcase pressure can cause oil leaks. The engine crankcase is designed to operate below atmospheric pressure, and the breather valve or pcv (positive crankcase ventilation) valve is the device used to keep crankcase pressure below atmospheric pressure. Excessive blow-by, a blown head gasket, a faulty breather valve, or a plugged breather hose can cause crankcase pressure to go beyond the working parameters of the breather valve, and cause crankcase pressure to exceed atmospheric pressure.

Crankcase pressure is constantly changing, and the primary reason for this is crankcase volume is costantly changing. The bottom of the engine piston and cylinder walls is part of the crankcase area. When the piston moves from BTC bottom dead center to TDC top dead center the bottom of the piston exposes more of the cylinder wall. This increases the crankcase area and lowers the crankcase pressure. When the piston moves from TDC to BDC the bottom of the piston exposes less of the cylinder wall, and this lowers the crankcase area and increases crankcase pressure. Another factor effecting crankcase pressure is Blow-by. Blow-by is when combustion gases leak past the seal created by the piston, piston rings and cyliner wall. These blow-by gases move past the seal and into the crankcase area, and this will increase the crankcase pressure. Blow-by is normal, but becomes worse when the engine is cold and when the engine is worn. Excessive blow-by can overcome the breather valve and crankcase pressure will rise above atmospheric pressure. 

The area of the crankcase extends up through the engine casing and underneath the valve cover on a OHV engine. If the part of the head gasket that seals the combustion chamber from the crankcase area blows, than the combustion gases will escape past the gasket and into the crankcase, and will raise crankcase pressure above the atmospheric pressure.

A leak down test can be used to test for a worn top end and leaking head gasket.

Check your engine service for how to test the breather valve on your engine, or you can view my YouTube video about testing breather valves if it relates to your engine:

Thanks for watching!
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