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The process of combustion forms several gases and vapors; many of
them quite corrosive. Some of these gases get past the piston rings
and into the crankcase. If left in the crankcase, these substances
would cause all kinds of bad things (rust, corrosion, and formation
of sludge), so they have to be removed. Back in the old days, they
used to be dumped out into the atmosphere through a tube. Once we
realized what a problem pollution was in the sixties, the PCV
(Positive Crankcase Ventilation) system was developed to take the
place of the old "dump tube."
The PCV system uses a hose connected between the engine and the
intake manifold to draw these gases out of the engine's crankcase and
back into the cylinders to burn with the regular fuel. The only
problem to solve is how to keep these gases from going willy-nilly
into the manifold and upsetting the required air-fuel ratio. The
solution to this problem is the PCV valve.
The PCV valve controls the release of crankcase gases and vapors to
the intake manifold. The valve is kept closed by spring action when
the engine is at rest. When the engine is running normally, the low
vacuum it creates allows the valve to open and release crankcase
vapors and gases into the intake manifold for burning. If the engine
is idling or you are slowing down, the vacuum level rises and pulls
the valve plunger into the valve opening. This partially blocks off
the opening so that only a small amount of vapors and gases can be
drawn into the intake manifold.
One really comforting feature of the PCV valve is its behavior in the
event of a backfire. If your car backfires in the manifold, the
pressure makes the spring close the valve completely. With the valve
closed, there is no chance that the flame can move into the crankcase
and cause an explosion.
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