Question:What is the PCV valve and what does it do?
Answer:The blowby vapors that end up in an engine's crankcase
contain moisture as well as combustion byproducts and unburned
fuel vapors. The crankcase is sealed to prevent the escape of
these gases into the atmosphere, but the vapors must be removed
to prevent oil contamination that leads to sludge formation.
The positive crankcase ventilation (PCV) system siphons these
vapors from the crankcase and routes them into the intake manifold
so they can be reburned in the engine.
The main component in the PCV system is the PCV valve, which
is usually located in the valve cover. A hose connects the PCV
valve to the intake manifold. A second hose between the air cleaner
and crankcase or other valve cover (V6 or V8 applications) provides
fresh air to help flush the vapors out of the crankcase. Some
engines have a separate air filter for the PCV breather hose located
inside the air cleaner.
The PCV valve is a spring-loaded valve with a specific orifice
size designed to restrict the amount of air that's siphoned from
the crankcase into the intake manifold. This is necessary because
air drawn through the valve from the crankcase has a leaning effect
on the fuel mixture much the same as a vacuum leak. So air flow
through the valve must be controlled within certain limits. At
idle, air flow is reduced because little blowby is produced.
When the engine is cruising and vacuum is high, airflow through
the PCV valve is at a maximum to purge the blowby vapors from
the crankcase.
It's important to note that PCV valves are sized for specific
engine applications. The wrong PCV valve for an application can
flow too much or too little air causing driveability problems.
Varnish deposits can clog the valve, so replacement for preventative
maintenance is recommended (every 50,000 miles usually).
Not all engines have PCV valves. Some (like Ford Escort,
GM FWD cars with the Quad Four engine, etc.) ventilate the crankcase
with a small breather hose and calibrated orifice. There is no
spring-loaded PCV valve. On these applications, no maintenance
is usually necessary.
