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PCV Valve Operation - Positive Crankcase Ventilation Valve.


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This is my BH5B PCV valve, mounted in the intake manifold, directly underneath the throttle body. Some WRX and other models, have the PCV valve mounted into one of the intake manifold runners:


PCV - Positive Crankcase Ventilation

PCV Valve - Positive Crankcase Ventilation Valve

PCV Valve removed:


The PCV Valve can be removed difficulty, or remove the TMIC for better access. TMIC removal covered here:


I've removed my PCV valve and tested it's operation, during manifold vac/boost. 19mm ring spanner to remove it.

When first removed, the valve was leaking slightly under boost. So technically a manifold boost leak, straight into the crankcase (through the block breather/PCV system). The small one way check valve inside seemed sticky, or gunked up.

I sprayed carb cleaner into the valve, blocked the ends with my fingers and shook it. Then repeated, until clear fluid came out. The check valve (sliding piston) now moves completely freely, I can hear the small spring inside it bouncing as the valve opens. It now seals completely under boost.

The block breather on my TT has an 'F' shaped plastic hose junction, right at the main block/crankcase breather. Which connects the crankcase breather to the PCV valve and the intake pipe:


When the PCV valve is closed crankcase gasses are drawn straight to the intake pipe, which is a constant low pressure or vacuum source. The higher the engine RPM the greater the intake pipe vacuum. During low engine speeds (idle/decelleration) when intake air speed/vacuum is low the PCV valve opens giving manifold vacuum to the PCV system.

Low pressure or vacuum in the crankcase removes condensation and hydrocarbon build up which lowers oil contamination, keeping the engine oil cleaner for longer. Crankcase vacuum also helps the piston oil ring seal against the bore, which lowers oil consumption.

I currently have my PCV valve blocked off, with the crankcase breather port connected straight to the intake pipe. But plan on rerouting the PCV valve port through an oil catch can, then to the intake manifold.

Edited by Rosssub
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 kamineko said:

i agree. if the pcv valve is in good condition you can use a catch can in between the crank-case and valve, and another between the rocker cover breathers and intake if you\'re worried about oil

this thread has motivated me to check my pcv though :)

New PCV Valves are cheap too. :)

  • Like 1
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 jesus said:

Not when you own an EZ. Or you have a warped definition of cheap.

I can replace mine for $12.00 plus GST. ;D

Block it off, or buy after market and fit one Inline?


Maybe gut your Original Valve and using it as a hose Fitting?

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[quote name='soopersubaru said:

Mmmm. to comment or not? Mmmm.


you know you can contribute to a discussion with your ideas or opinions, just leave out the bullshit

pointless either way. or increased points.

Ive done this pcv stuff.Measured crankcase pressures etc. The crap created in the crankcase should be removed. Gas and fluid.

Pcv fitted to make sure liquid waste does not end up hitting the turbine blades (Turbo cars)

here is some reading on how to:- http://www.clubsub.org.nz/forum/index.php/topic,42008.msg534608.html#msg534608

There ya go! A sensible comment. At minus 11. Now lets see what transpires.

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 boon said:

How would liquid waste make its way from the crankcase to the turbine blades?

Intrigued to know.

(Unless you mean compressor wheel?)

MY error. Yes compressor turbine wheel specifically.(Inlet)((Where the air enters the turbo))

Ya dont want debris Either liquid or solid going in there! AT ALL.

Standard PCV is also a restriction by design. Not specifically a spring loaded non return valve.

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  • 10 months later...

Only an issue if it's leaking under boost really, gunked up and not sealing properly. Then it's a boost leak from the TB straight into the crankcase.

Easiest way to check it without removing it would be the old 'blocked intake boost leak test'. You'd hear air escaping from the TB into the crank case.

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Quick question relating to the PCV valve (non turbo sorry). In the week end while taking off the torque box I ended up snapping the torque box hose connection on the PCV, at the moment I have just plugged it up, will this cause problems? Should I be getting a new PCV or will just blocking it be ok?

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So if one was to be removing the TMIC, to replace dodgy old intake hoses with silicone, then it would be a good idea to clean the PCV valve out while its off yah?

Having had a sqizz at it, I have no idea how you'd get at it without taking the TMIC off, unless you have hands and arms like an octopus... which I don't... mine are more like sausages...

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