Help me understand how the PCV system works.

[vrg3]

Nico, I know you asked me directly about a catch can installation in another thread, and I wasn't trying to ignore your question. I was actually trying to figure out the answer. I haven't been able to.

Let's figure out how exactly the PCV system in our cars works. This has been bugging me for a long time. With all our heads together we should be able to straighten it all out.

1) There is a line from the intake pipe that splits and leads into both valve covers:

On non-turbos, this is the upstream fitting on the black plastic intake pipe that's pointed to the driver side. It connects to a tee built into the pipe, and each of the other ends of the tee go off to the valve covers.

On turbos, this the upstream fitting on the compressor inlet elbow that's pointed towards the front of the car. It has a black hard plastic splitter coming off it that yields approximately equal-length paths to both valve covers.

2) There is another line from the intake pipe that splits and leads to the PCV valve and to the large fitting on the back side of the middle of the crankcase, slightly downstream of the first fitting:

On non-turbos, this is the fitting on the underside of the black plastic intake pipe that's pointing to the passenger side. A hose goes from there to a weirdly shaped tee. The bottom of the tee has a short hose connecting it to the crankcase, and the top is connected to the hose leading to the PCV valve.

On turbos, this is the fitting on the compressor inlet elbow that's facing the driver side. A short hose connects it to the F-shaped plastic. The other ends of the F connect to the crankcase and to the hose going to the PCV valve.

3) On the turbos, there is an extra fitting on each valve cover and one on the block underneath the alternator. These are connected with a long plastic tee.

So these are my guesses:

The line that splits leads into the valve covers brings fresh air into the system.

Normally, when manifold vacuum is high, the fresh air goes through the valve covers to the crankcase and picks up blowby along the way. This blowby then goes up through the fitting on the crankcase, and intake manifold vacuum sucks it through the PCV valve.

When manifold vacuum is low (or nonexistent when you're on boost), the blowby instead gets sucked in by the partial vacuum in the intake pipe. This, then, is where you'd want to stick a catch can to keep excessive oil out of the intake.

I don't understand why the turbo has the extra pipe to move vapor between the crankcase and heads. Maybe the closed deck block means that vapor can't move as easily through the oil passages?

Please correct me if I've made any mistakes in the above paragraphs. But if I haven't, I still have a bunch of questions -- how do the gases know what to do? How do they know to enter the valve covers and leave the crankcase as opposed to the other way around? And why isn't the PCV valve a big boost leak when you're on boost? How critical are the exact shapes and sizes of the hoses and plastic pipes? And, and .... stuff like that.

[evolutionmovement]

Doesn't the PCV valve only open under vacuum? If so, under boost it would shut the valve.

Steve

[vrg3]

From my understanding, it's actually the other way around. The higher the manifold vacuum, the more it closes. That prevents blowby from screwing up fuel mixtures at idle.

[NuwanD]

The point of a PCV (positive crankcase ventilation) system is to prevent excess pressure from developing within the crankcase from blow-by gases... without it the pressure would blow every seal in the motor...

That being said, on a turbo car the pcv valve would be open under boost as under boost the most blow-by should be present, on an n/a car the same goes minus the boost.... if pressure develops the pcv valve opens and releases the pressure along with any oil/fuel vapour back into the intake stream which also provides better emissions and fuel mileage.

I believe a little pressure can develop in the valve covers especailly on a turbo car where high pressure (boost) is present in the intake ports, which could seap through the valve seals... that's why we have that;

extra fitting on each valve cover and one on the block underneath the alternator. These are connected with a long plastic tee.

... which would allow pressure to equalize between the crankcase and valve covers, and since the crankcase pressure is vented by the PCV valve the valve covers are vented as well, without the need for a valve for the valve covers... i guess it would have been called the "PVCV" (positive valve cover ventilation) valve.

vrg3; I hope that makes sense, in regards to the lengths and sizes of piping in the system.. i don't believe that they are critical, as long as they are not unreasonably long and won't collapse on themselves i don't see a reason to be worried when altering the system... look at all those aftermarket intakes which come with lengths of silicone to reconnect all the emissions lines... they don't seem to hurt anything

[vrg3]

Well, there are two purposes of PCV, right? One is to reduce the pressure differential across the rings like you say, but the other is to prevent crankcase fumes from escaping to the atmosphere. Old systems which only did the first just had an inlet on the crankcase and another orifice at some position where passing air would draw fumes out via the venturi effect.

I understand and agree with what you're saying about the valve opening to relieve pressure in the crankcase, but my question is -- wouldn't it also relieve manifold pressure back to the compressor inlet? Is it just a small enough opening that it doesn't matter?

I guess I can understand how the heads could be more pressurized than the crankcase as you describe. So then it's true that gases can't really move freely between the crankcase and the heads?

I realize lots of aftermarket intakes change the lengths and sizes and stuff, and part of what I'm asking is whether or not they actually interfere with the stock PCV configuration at all. It's possible that they do and nobody's really noticed, right? Their blowby could be primarily going into the intake tract instead of the manifold and they might not really notice the extra oil coating everything, or they might have put a catch can in which might mask some symptoms.

The only answer I can think of to my question about how the gases know where to go is that somehow the precise locations, sizes, and lengths of piping make it happen.

[evolutionmovement]

Forget it. I don't know why the hell I was thinking it opened under vacuum. I think I was picturing it with the valve closed at rest for some reason. That's what happens when you try to do several things at once.

Steve

[Legacy777]

I don't have much to contribute on the terms of analysis right now....brain doesn't want to think about it. But I do have a diagram for the N/A motors.

http://www.main.experiencetherave.com:8 ... system.jpg

[Legacy777]

The description described on that page is pretty decent IMO

[vrg3]

The description on that page seems mostly consistent with my guesses, but it still leaves me confused about my questions.

It also seems to indicate that fresh air flows out of the intake tract through both fittings and not back, which might be true sometimes but not always or something.

Here's the turbo version of the diagram:

http://www.graphics.cornell.edu/~v/pics/turbo_pcv.png

[NICO]

ok this what i no and have tested and done before i blew up my piston.

i had the crank case hose on a catch can and stright to the pcv valve, the valve cover lines would be on a single k&n little air filter.

this set up would let me run crazy boost like 18psi to 20psi, also the car would rev freely its like there was that much boost but there was not that much boost smooth reving.

ok this was becuse i had no clue i ran the valve cover line to the pcv and the crank case line i put on a catch can (that line all ways spits out oil) stright to a k&n little air filter and the oil would some times spit out cuz the catch can would be a little full.

but this set up gave me less boost but crazy take offs and short burst of hard low boost lets say 15psi.

also dont think, well this why his motor failed its not it had 500thousand ks, it was the way i drive and it was about that time, 7 years of hard tuning and driving

also i would change that set up very often trying to get more power out the car, now we just rebuilt it so i looked in the book and i was holy @#$! i had everything way wrong, all that just to try and stop the oil going in to the turbo.

i really think it comes from the crank case line and i am going to put the crank case line direct to the catch can then to where there spots where from the factory.

[NuwanD]

Well, there are two purposes of PCV, right? One is to reduce the pressure differential across the rings like you say, but the other is to prevent crankcase fumes from escaping to the atmosphere.

yeppers, pressure and re-burning vapours

but my question is -- wouldn't it also relieve manifold pressure back to the compressor inlet? Is it just a small enough opening that it doesn't matter?

I think what we have to look at is the fact that the PCV valve opens and closes dependant on the pressure difference on either side of the valve... if the intake manifold is seeing high boost and there is some pressure from blow-by in the crankcase then the valve is closed, when you shift and subsequently let off the gas the valve would open (vacuum in intake manifold and bypass gas pressure in crank)... i guess my earlier statement of it being open on boost was incorrect... the PCV valve must flutter often between open and closed as driving loads and throttle positions change... most likely acting somewhat similar to a blow off valve?

So then it's true that gases can't really move freely between the crankcase and the heads?

Well i guess that depends on how you look at it, with the addition of that line that goes from either head to the crankcase, yes... but looking at strictly the heads and block i don't see any point from which gases can travel freely... there are only 3 types of areas on the mating surface between block and head; oil passage/coolant passage/combustion chamber.... and of the three only the combustion chamber (pistons and valve) could possibly transfer gases, although I would look at it more as seaping and bypass gases.

I realize lots of aftermarket intakes change the lengths and sizes and stuff....

After thinking about it more it seems that aftermarket intakes which should flow better and thus create lower pressure do tend to cause more oil eating issues especially when driven hard, as is the case with a couple of friends with 2.5RS's and intakes

The only answer I can think of to my question about how the gases know where to go is that somehow the precise locations, sizes, and lengths of piping make it happen.

Location of pipes definately do matter, but i think this is more of a case of high to low pressure movement rather than trying to direct gases despite pressure differentials... I would like to think that diffusion of the gases occurs from low to high concentration, but i believe that gas production and evacuation within the motor occur to quickly for diffusion to take place.

[vrg3]

I think what we have to look at is the fact that the PCV valve opens and closes dependant on the pressure difference on either side of the valve... if the intake manifold is seeing high boost and there is some pressure from blow-by in the crankcase then the valve is closed, when you shift and subsequently let off the gas the valve would open (vacuum in intake manifold and bypass gas pressure in crank)... i guess my earlier statement of it being open on boost was incorrect... the PCV valve must flutter often between open and closed as driving loads and throttle positions change... most likely acting somewhat similar to a blow off valve?

But the line going to the PCV valve also goes to the compressor inlet, and manifold pressure is certainly higher than compressor inlet pressure, right?

But now we get into the issue of sizes. The bottom of the F (that goes to the crankcase) has a 19mm OD, while the end going to the compressor inlet has a 12mm OD. The end going to the PCV valve has a 15mm OD. I'm pretty sure the sizes are the same on the corresponding parts of the non-turbo's tee too. And for all I know there might even be restrictors and stuff inside the plastic F or tee.

I would guess the valve would spend most of its time neither open nor closed but rather at some equilibrium position in between, except that it's odd because it seems to be a positive feedback loop, which wouldn't yield an equilibrium: positive pressure differential between the valve inlet and the manifold causes the valve to close more, which ought to increase the pressure differential.

but looking at strictly the heads and block i don't see any point from which gases can travel freely... there are only 3 types of areas on the mating surface between block and head; oil passage/coolant passage/combustion chamber.... and of the three only the combustion chamber (pistons and valve) could possibly transfer gases, although I would look at it more as seaping and bypass gases.

I thought the big oil passages on the bottom of the mating surface lead directly to the sump, and that there was a lot more room there than would be needed for simple oil flow. After all, the turbo oil drain leads to this area of the head, right?

And that seems consistent with the diagrams Josh and I posted.

After thinking about it more it seems that aftermarket intakes which should flow better and thus create lower pressure do tend to cause more oil eating issues especially when driven hard, as is the case with a couple of friends with 2.5RS's and intakes

Hm! Eenteresting.

Location of pipes definately do matter, but i think this is more of a case of high to low pressure movement rather than trying to direct gases despite pressure differentials... I would like to think that diffusion of the gases occurs from low to high concentration, but i believe that gas production and evacuation within the motor occur to quickly for diffusion to take place.

Yeah, I don't think diffusion really gets much of a chance to do its thing here... I agree that it's gotta be pressure differentials doing everything. This can include the venturi effect.

I just remembered something -- I think one of the plastic pipes going to the compressor inlet has a special shape. It's like closed on the top and open on the bottom or something. Kind of like if you took a tube and cut it at a sharp angle. I don't remember which pipe it was though; I think it may have been the one from the compressor inlet to the F-shaped pipe. Maybe that shape causes something to happen too.

[NICO]

where can i put my catch can, is it fine to have it on the crank case or the valve covers, which one spits out the most oil in to the turbo or is it both coating it, you no those little k&n air filters, where can i put one of those on my car every pic i c every one has one, where do they fit on a subaru or we dont need them.

[myles]

Taken from the FSM for 2002 Legacy (USDM):

EC-6 3. Crankcase Emission Control System

The positive crankcase ventilation (PCV) system prevents air pollution which will be caused by blow-by gas being emitted from the crankcase. The system consists of a sealed oil filler cap, rocker covers with fresh air inlet, connecting hoses, a PCV valve and an air intake duct.

In a part-throttle condition, the blow-by gas in the crankcase flows into the intake manifold through the connecting hose of crankcase and PCV valve by the strong vacuum created in the intake manifold. Under this condition, fresh air is introduced into the crankcase through the connecting hose of the rocker cover.

In a wide-open-throttle condition, a part of blow-by gas flows into the air intake duct through the connecting hose and is drawn into the throttle chamber, because under this is condition, the intake manifold vacuum is not strong enough to introduce through the PCV valve all blow-by gases that increase in the amount with engine speed.


Is there any way to upload the .pdf? It has a few nice diagrams.

[vrg3]

Unfortunately Josh's server doesn't seem to be working... that is almost word-for-word the same text as in the Legacy service manual that Josh scanned a page from.

You can't upload attachments here; you need to put them on a server elsewhere and link to them. You may be able to find someone to volunteer to host it for you.

I don't think the FSM answers the questions, though. I've read through several FSMs for various years and models and they all say this exact same thing. They don't explain the mechanism by which it's ensured fresh air goes in the right place and how they keep the PCV valve from being a boost leak and stuff like that.

[BAC5.2]

http://www.misterfixit.com/pcvvalve.htm

I don't understand why the turbo has the extra pipe to move vapor between the crankcase and heads. Maybe the closed deck block means that vapor can't move as easily through the oil passages?

It would appear that the second pipe would be to bleed off excess vapor that the PCV valve can't handle. Under no boost (but no vaccum, i.e. N/A application), the valve would open (high vaccum closes it like Vikash said to keep fuel mix constant), and allow any blowby to exit. Under boost, the valve would already be open to it's fullest, so it couldn't flow any more, yet blow-by would be increased thanks to the increased pressure (boost). The second pipe likely helps handle this.

[vrg3]

I thought PCV was named after a guy named Positive.

How would moving vapors between the heads and crankcase relieve any pressure?

I just thought of one reason that EJ22Ts would have more trouble than EJ22s moving gases between the heads (well, the passenger side head anyway) and the crankcase -- the turbo oil drain line goes into the passenger side head. All that froth probably gets in the way.

[Legacy777]

should be working now. rr was having issues today

[myles]

If the pressure in the intake manifold is greater than that in the crankcase (such as would occur under boost), the valve will close.

The air that's vented from the crankcase via the PCV will be replaced by fresh air via the breather tubes in each valve cover (the other end of these tubes connect to the intake tract post-air filter).

[vrg3]

By what mechanism will the valve close when manifold pressure is higher than crankcase pressure? It normally just closes when the maniold is at relatively lower pressure than the other side of the valve.

[NICO]

so how should i put my catch can on. crank case to catch can then back to where they go one to the pcv and in the air strem to the turbo, does it sound right.

or should i put the catch can on the vavle cover lines and then back in the intake.

also what can be open air with those little fillters.

[Wedge]

I've been monitoring my engine, and it seems that MOST of the blowby oil comes from the vent that is located under the alternator.

-Vic

[vrg3]

That vent only goes to the heads, though, not to any point on the intake system.

[Wedge]

Ahhh, well that explains it, since I've got it hooked to the PCV system.

Now I need to figure out how the heck am I going to route it to my SOHC RS heads.

[vrg3]

Maybe you could weld 1/2" pipe fittings onto the valve covers? Or if the material is thick enough, maybe even just drill and tap the covers? FittingsAndAdapters.com sells nice steel 1/2" beaded hose barbs with 3/8" pipe threads on the other end for less than 2 bucks apiece.