Is it like a Check valve? Does it open up when there is a lot of pressure in the crankcase? Or does it open up from Intake manifold vacuum?

 

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.

HTH

 

BSQ to the rescue. Maybe you should change your name to PCV?

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BG that diagram shows what I suspected....under boost the valve will close. I would think that the valve would be strong enough to not blow under boost. In any case, I would keep and eye on it, and replace it at every oil change, or every other oil change (which is what should happen anyway). I have cleaned mine with Carb Cleaner (or similar) in between changes...it can get pretty clogged up.

 

i'm still confused. does thte pcv valve NEED to see vacuum on the intake manifodl side to open and vent the crankcase vapors? or will it open anyway when it senses pressure inside the crankcase? (i'm trying to decide exactly how to set up my catchcan, have heard many different opinions.)

 

The PCV valve operates on vacuum for the intake manifold only.

An oil catch can should be hooked up between the PCV valve and the intake manifold. Here is a diagram which shows how an oil catch can should be hooked up.

 

so when the motor is under load, out if vac, or in boost, the pcv valve will be totally closed? in this case it would seem that the crankcase pressure would have nowhere to go except out through the valve cover breather?

some ppl run a tube from the pcv valve to open atmosphere. from your explanation it would seem that the pvc valve would never open becasue it would never see vacuum. what happens if you eliminate the pcv valve and just vent the gases continuously, into a catchcan w/ a filter or into open atm. as some ppl do...?