Modern PCV systems do not require a source of fresh air. Typically they
have a hose from the engine that goes through an oil air separator and then
usually it splits off with a small hose to the intake and a large hose to
the intake in front of the throttle plate.
Contrary to what people have been posting the large hose is not a fresh air
intake.
At idle there is a small amount of blow by and it is drawn into the intake
via the vacuum hose.
At large throttle openings, the amount of blow by would overwhelm the small
hose going to the intake (plus on a turbo, the intake is under pressure),
but the large hose is a slightly lower than atmospheric pressure due to the
venturi effect of the air flowing into the engine. The blow by then flows
through the large hose and through the engine that way.
Some modern systems use a diaphragm to modulate the crankcase pressure at
an even lower level than just the small hose into the intake can provide.
One of the checks that technicians do on modern cars is to check the
crankcase pressure at both idle and 2500 RPM. With a properly operating
PCV system, the crankcase will run at a very slight negative pressure under
both conditions. A marginal system will be a zero pressure/vacuum, and a
plugged system will run a positive pressure.
Positive pressure is very bad in a crankcase (any crankcase, PCV or not)
Build just a tiny bit of positive pressure and you can create all kinds of
oil leaks (the pressure will get out, usually taking oil with it), and even
in some case blow a seal clean out of the engine.
Positive pressure in a crankcase is very very bad. You don't ever want
positive crankcase pressure on any engine.
Rick
On Wed, Feb 1, 2012 at 1:54 PM, Bob Spidell <bspidell@comcast.net> wrote:
> Almost fired off a flame, then realized you're (probably) referring to the
> fresh air source that typical PCV systems require (hence the word
> 'system').
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