A breather pipe allows an internal combustion engine to vent crankcase pressure out of the engine. Despite the name, it is actually a vent pipe. By venting pressure out of the crankcase, an engine's piston rings are allowed to seal tighter against the cylinder walls. This provides better compression and promotes less oil consumption while making more horsepower.
This pipe often is attached to a valve cover, and it is often recognized as the location to pour oil into the engine. Often, a breather pipe will incorporate a paper filter that is designed to catch any oil blow-by that may occur. In high-performance engines, dual pipe configurations are common.
When the crankshaft and pistons rotate within the engine block, a great deal of air is whipped into the engine. A combustion engine is in reality nothing more than an air pump. If the air is left inside of the engine, the oil would become frothy and would lose its lubricating properties. The air would also have to try to escape out of the engine as it built up pressure.
Without a vent, the built up air would attempt to escape around the pistons by seeping past the piston rings. This would cause a drastic loss in compression and power. The engine's oil would also be allowed to escape around the piston rings, causing a lot of smoke, fouled spark plugs, and potentially burned pistons.
The breather pipe works well for the family car, but racing engines require added venting and often incorporate an air pump to pull the air out of the crankcase. This vacuum pump draws more air out of the engine than can be vented through a pipe. This creates a very sound running engine with little worry about ring sealing or oil leaking into the combustion chamber.
When an engine suffers a clogged or plugged breather pipe, the engine runs hot and will often begin to misfire. Oil consumption will increase and the engine will respond like it has less power. Failure to repair the plugged breather can lead to fouled spark plugs and possibly a blown head gasket or intake gasket as the built up air attempts to escape the engine.
Proper breather maintenance can prevent a major engine catastrophe. To keep it working properly, car owners should clean the engine's breather at every oil change with hot soapy water and allowing it to air dry. A lost or misplaced breather cap should be replaced as soon as possible to prevent debris from entering the engine, and will help prolong its life.