PCV System Question

[kschachn]

The PCV system takes air from the air intake, runs it through the valve cover(s), through the PCV valve, and then into the intake manifold, right? So if you have a V-engine (V6 in my case) how are the two cylinder head covers connected? I don't see a tube or hose between the two.

Also, if you take the hose off the air intake and off the manifold, you should be able to blow into the intake hose and have it come out the manifold hose, right? This would tell you if the airflow has an open path or not?

 

[bepperb]

No. When your cylinders fire some of the gas gets past the piston rings and into the crankcase. This gas needs to be vented, and in the old days was just vented to the atmosphere. The PCV is a one way valve out of the engine and into the intake.

 

[sven98]

On a V-style motor, the PCV takes fresh air in on one valve cover, and draws air out (blowby gasses) the other valve cover.

 

[mechanicx]

Originally Posted By: kschachn
The PCV system takes air from the air intake, runs it through the valve cover(s), through the PCV valve, and then into the intake manifold, right? So if you have a V-engine (V6 in my case) how are the two cylinder head covers connected? I don't see a tube or hose between the two.



Under the valve cover are oil drainback holes that usually go to the valley and drain back into the crankcase. But lets assume the valley was completely dry and sealed off for the cylinder heads. The opposite valve cover still has a path through oil drain back holes to the crankcase and on to the other valve cover.

 

[bepperb]

"On a V-style motor, the PCV takes fresh air in on one valve cover, and draws air out (blowby gasses) the other valve cover."

No.

 

[oilslick44]

The PCV valve routes blow by gases from the crankcase to the intake to be burned. It's a closed system, I don't see where you
think fresh air is coming in.

 

[mechtech2]

Some PCV systems take air from the intake.
This never made sense to me.
It should not be in the intake that is metered.
It should be in FRONT of the air cleaner.

But normally:
In one valve cover, through the engine, and out the other valve cover to the PCV valve and back into the intake manifold .

 

[kschachn]

Yeah, it is a hose between the air intake and the front valve cover. It is indeed after the mass airflow sensor, so it is metered. I guess they figure that since it ends up in the intake manifold it should be accounted for. IDK.

FWIW, the system in question is on a Toyota 1MZ-FE.

Oh, and thanks for the replies. I forgot that it is a positive crankcase ventilation valve, that makes sense then that it is there where the "connection" is made between the cylinder banks.

Originally Posted By: mechtech2
Some PCV systems take air from the intake.
This never made sense to me.
It should not be in the intake that is metered.
It should be in FRONT of the air cleaner.

But normally:
In one valve cover, through the engine, and out the other valve cover to the PCV valve and back into the intake manifold .

 

[sven98]

Originally Posted By: bepperb
No.


Yes. Here's a typical setup...

 

[kschachn]

Oh, and BTW - I can blow into this hose and have it come out the hose leading into the intake manifold.

This started because I was wondering about the condition of the baffles in the valve cover, as to how "open" or "closed" they were. On the basis of my "blow test" they seem very open. No airflow restriction could be felt.

Nothing felt, but a bit of oil tasted ;-)

 

[SuperBusa]

Originally Posted By: sven98
Originally Posted By: bepperb
No.


Yes. Here's a typical setup...

As the schematic shows, on a modern engine the fresh air is typically taken at a point after the MAF sensor and before the throttle valve - it's "metered". The "dirty" vapors (blow-by, etc) are sucked into the intake manifold via the PCV valve to be ingested and burned in the engine.

 

[Kiwi_ME]

It's important to realize that the vent to the pre-throttle intake tract flows both ways. A good description is found on Wiki:
Originally Posted By: http://en.wikipedia.org/wiki/PCV_valve
The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.