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Old 04-13-2009, 01:23 AM   #1
williaty
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Default Why Do NA Cars Even Have PCV Valves?

I get why they have a PCV system but I can't figure out why they need a PCV valve. First, look at this diagram of the PCV system in a 2005 2.5RS so we're all on the same page:


OK, everybody back? Now, as you can see, the air enters the big red assembly on the top left. This block represents the "torque box" and accordion intake tube all in one. As you can see, the valve cover breather lines are attached here. Keep in mind that vacuum in this part of the system is very minimal, just the tiny bit caused by the restriction of the air filter. Heading on towards the engine, you can see that the PCV return line is connected to the intake manifold. Keep in mind that the vacuum at this point in the system varies depending on the position of the throttle, but with the throttle mostly closed, the vacuum here can become very strong. OK, last thing to notice, the air in the valvecovers is connected to the air in the crankcase via the oil return galleries between the heads and the crank.

Alright, put it all together and how does it work? Well, it works off the vacuum difference between the point in the torque box where the valvecover breather lines connect and the point on the intake manifold where the PCV return line connects. Since the average pressure in the intake manifold will always be as low or lower than average pressure in the torque box, the net flow is always going to be from the torque box to the intake manifold.

What does this mean in terms of the PCV system? Outside air passes through the air filter and into the torque box. The air then goes into the valvecover breather lines (in other words, the flow is opposite the direction everyone usually assumes). From the valvecover area, it passes through the oil return galleries into the sump and crank case. From the crank case, it passes through the PCV valve and return line and into the intake manifold. The contaminated PCV gasses are burnt in the engine and then shot out the tail pipe with the rest of the exhaust.

So, the PCV valve is a 1-way flow control valve. It's open any time the pressure in the crankcase is higher than the pressure in the intake manifold. Now, if you think about it for a moment, this is always going to be true when the engine is running. So what on earth did Subaru include a PCV valve for if it's always open?



EDIT:It's very important to note that the situation I described is only true for lower RPM, lower throttle operation. At high-RPM, WOT operation, the crankcase pressurizes (from blowby) to the point that the PCV valve can no longer flow enough volume to de-pressurize the crankcase. Once this happens, the flow direction through the heads and valvecovers reverses. That means that, at high RPM and WOT, the flow in the valvecover breathers reverses and begins to blow out into the intake. As soon as you get off the gas and the engine slows down, the overpressure in the crankcase drops to the point the PCV valve can handle it and the flow in the valvecover breathers returns to normal (into the engine). However, as stated above, the PCV valve is always flowing out from the crankcase to the intake manifold, regardless of throttle position or engine speed.
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Last edited by williaty; 05-14-2009 at 03:43 AM.
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Old 04-13-2009, 01:27 AM   #2
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So that the flow always remains one way. I see your point, and can't say that I disagree.
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Old 04-13-2009, 08:41 AM   #3
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While the PCV valve is always ďopenĒ (At least partially), the amount to which it is fully open varies depending on the level of vacuum. At idle, the PCV valve is as close to fully closed as possible, and it opens more as engine load increases. So first of all, itís not always completely open.

Like you mentioned above, itís a one-way valve, so flow can only go in one direction: From the crankcase.

In the case of an engine backfire, for example, this can be useful to prevent gases from going the wrong way, that being back into the crankcase.

So, think of your PCV valve as being cheap (Under $10) insurance in a scenario such as that, unlikely as it is if everything is working properly.


But the real reason for the PCV valve is that The Man (i.e. Subaru) is trying to slowly take all of your money by making you buy these stupid things every 15,000-20,000 miles.
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Old 04-13-2009, 09:41 AM   #4
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That is a nice colorful diagram. The check valve (PCV) can only do is to prevent a back airflow.

That being said, I have never felt a vacuum off of any of the breather tubes when the engine is running. Not like what this diagram indicates. If anything, we catch oil out of the breather line or find oil residue in the intake track coming out of the breather, correct? Doesn't that mean air actually comes out of the breather despite of the diagram? This is the case, based on the diagram, when the PCV is closed, creating positive pressure inside.

Why is the PCV closed (or almost closed). I think it has a spring inside like a typical check valve. To activate (open) it, the Vacuum in the manifold has to 1st overcome the spring. If the PCV is stuck closed (contaminated) or the vacuum in the manifold is not strong enough to pull the spring, it will remain closed. And the idle often suffers for whatever reason.

I don't think a strong vacuum in the manifold is good as we strive to reduce it by having better air intakes. With a very good intake, there is only high air velocity in the manifold not high vacuum, even at high rpm. If that is the case, the PCV may not be working as intended (open). So the effect of crankcase air "flow" is reduced and pressure inside increases, unless the PCV is gone, particularly, the spring inside PCV is removed.

To experiment, I guess one can simply gut one of these PCV valve by drilling out that ball, and find out if the car runs smoother without that retarded ball (). I also vote to get rid of it just keep the PCV shell.

Last edited by Jerry Xu; 04-13-2009 at 11:33 AM.
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Old 04-13-2009, 10:18 AM   #5
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Default pcv

The primary reason for the PCV valve is to regulate the PCV flow. At light load, the PCV valve regulates the crankcase ventilation based on the vacuum in the intake manifold, as you described. At light load the blowby rate of flow is relatively low, so this system works well with a properly calibrated PCV valve and can maintain a slight vacuum in the crankcase. Remember that the crankcase is pressurized by blowby gasses, and the rate is proportional to power, so the pressure in the crankcase varies a lot during normal driving. The PCV is a flow regulator that limits PCV flow under varying pressure differences and limits how much vacuum the crankcase sees. Some engines use an orifice instead of a PCV valve, but this is a cheap solution that over-ventilates the crankcase at some speed/laod points and under-ventialtes at others. Over ventilation tends to lead to higher oil consumption, since there is always some oil mist that gets carried over in the PCV flow. Also, an orifice is not as good in terms of allowing the idle speed control system to work well (too much flow). Without an orifice or a PCV valve there would more air pulled through the PCV path than is needed for idle speed.

At high load, with rather substantial blowby flow entering the crankcase past all of the pistons, the crankcase ventilation flow path reverses, and what was the air inlet path becomes the pressure relief path for all of that blowby. At this time the PCV valve acts as a flow limiter so that oil mist from the crankcase does not get pulled into the intake manifold. The PCV valve and path can not flow enough to prevent the crankcase from pressurizing and blowing out the crankcase seals. Also the PCV path can only handle a certain amount of flow before oil mist seperation gets poor. Oil seperation is designed into the connection between the valve cover and the air box, but it is not perfect and if you drive at high power a lot, you may see some oil in the air cleaner or zip tubes.

So don't modify the PCV system, a lot of engineering went into it. Also remember that the blowby gasses contain a high concentration of Hydrocarbons which is "fuel" that would otherwise be wasted.

I am an engine design and development engineer with about 25 years experience.


Dave

Last edited by dblotii; 04-13-2009 at 11:00 AM. Reason: clarification
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Old 04-13-2009, 10:41 AM   #6
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Quote:
Originally Posted by dblotii View Post
Oil seperation is designed into the connection between the vlave cover and the air box, but it is not perfect
This explains why the hose from the crank-case to the air-box is that silly "S" shaped hose.
It's a really really simple oil catch line. Air and gass can get pushed up into the air-box, but I suspect the S is suposed to catch most of the oil somehow.
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Old 04-13-2009, 10:57 AM   #7
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the newer imprezas like my '06 2.5i, don't have pvc systems. they just rely on the neg air pressure from the intake to maintain flow in the crankcase at high loads. the only problem is that if you have an aftermarket intake, you should have an air/oil seperator because the stock one is part of the crank case ventalation system. otherwise oil mist can get into the combustion chamber and cause knocking.
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Old 04-13-2009, 12:46 PM   #8
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Dblotii already posted the entire reason. This saves my time to explain it myself, Thankyou Dave.
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Old 04-13-2009, 12:54 PM   #9
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Quote:
Originally Posted by dblotii View Post
So don't modify the PCV system, a lot of engineering went into it. Also remember that the blowby gasses contain a high concentration of Hydrocarbons which is "fuel" that would otherwise be wasted.

I am an engine design and development engineer with about 25 years experience.


Dave
First of all, thanks for showing up with a good answer!

Second, I'm having truly excessive oil show up in the intake manifold via the PCV system. While the ultimate solution is obviously new rings to replace my worn out ones, I need something to bridge the gap between now and having the money to fix it properly. I'm looking to install an air/oil separator that reclaims the oil suspended in the PCV flow and returns the oil to the sump. Looking at commercial options, one recommended deleting the PCV valve and one recommended retaining it. I thought that I should probably understand what the PCV valve did before I made a decision about deleting it
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Old 04-13-2009, 01:17 PM   #10
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Quote:
Originally Posted by williaty View Post
First of all, thanks for showing up with a good answer!

Second, I'm having truly excessive oil show up in the intake manifold via the PCV system. While the ultimate solution is obviously new rings to replace my worn out ones, I need something to bridge the gap between now and having the money to fix it properly. I'm looking to install an air/oil separator that reclaims the oil suspended in the PCV flow and returns the oil to the sump. Looking at commercial options, one recommended deleting the PCV valve and one recommended retaining it. I thought that I should probably understand what the PCV valve did before I made a decision about deleting it
you can do one of 2 things. if you have a couple hundred bucks you can get the AVO (or comperable) oil seperator (http://www.rallitek.com/avooilsekit.html), or build one from pvc pipeing and some basic plumbing hardware (you can google it and there a couple sets of instrunctions floating around). if its really alot of oil and you need a bandaid for a day or two , you can always rig a large fuel filter on the outlet breather tube (or just disconect the tube from your manifold alltogether).
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Old 04-13-2009, 01:29 PM   #11
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I usually just put a breather filter right on the valve cover. makes a mess sometimes, but thats why you have a rag on ya! Lol.
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Old 04-13-2009, 01:50 PM   #12
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Quote:
Originally Posted by volgrath View Post
I usually just put a breather filter right on the valve cover. makes a mess sometimes, but thats why you have a rag on ya! Lol.
ya i did that for my old 89 z24...makes for a nice automatic oil-coating engine protector haha
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Old 04-13-2009, 01:55 PM   #13
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hey hey! dont knock it man! that nice oil coating keeps it from rusting! lolol XD
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Old 04-13-2009, 02:00 PM   #14
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haha it did do that quite well....
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Old 04-13-2009, 02:21 PM   #15
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My PCV valve is currently removed from my car.
I now just the valve cover and crank case hoses going to a single small PCV system air filter. And the PCV valve is gone, and that intake port is feeding my vac/boost gauge. Basically, any blowby just gets vented to atmo. .. Yeah.. I know.. the environment will hate me for it I have not yet done it, but will be putting a catch can in the line in the future. Since having this combo for just about a month now, I've not yet gotten any oil coming out the filter I also still have 200psi on all 4 cyls, so I'm getting minimal blowby too.
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Old 04-13-2009, 03:46 PM   #16
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Quote:
Originally Posted by treksofar View Post
the newer imprezas like my '06 2.5i, don't have pvc systems. they just rely on the neg air pressure from the intake to maintain flow in the crankcase at high loads. the only problem is that if you have an aftermarket intake, you should have an air/oil seperator because the stock one is part of the crank case ventalation system. otherwise oil mist can get into the combustion chamber and cause knocking.
My '06 2.5i has a PCV valve exactly like the OP's '05 RS.
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Old 04-13-2009, 11:50 PM   #17
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Another reason not to remove the PCv valve is that if you remove the valve and block the PCV valve connection, you aren't actually ventilating the crankcase with fresh air. Remember that blowby gasses contain water vapor, Hydrocarbons and NOx. The water vapor and NOx degrade the oil much faster if there is no PCV valve (or orifice) to continually purge these substances from the crankcase (whenever your throttle is not wide-open). Your oil will sludge up and oxidize much faster without a PCV valve. Also the HC's in the blowby gasses are fuel that is best burned in the combustion chamber rather than just wasted.

Dave
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Old 04-13-2009, 11:57 PM   #18
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Oh, neither vendor recommends plugging the port, just eliminating the valve and routing it through the air/oil separator instead.
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Old 04-14-2009, 01:09 AM   #19
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Then to the manifold for further burning.
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Old 04-15-2009, 09:48 AM   #20
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Quote:
Originally Posted by semaj View Post
My '06 2.5i has a PCV valve exactly like the OP's '05 RS.
i stand corrected. when i was installing my sri, i was informed that there was something different with the pcv valve on the '06 then the rs and it stuck with me. then again this is my first subbie and the motor is a way different beast then what i'm used to.
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Old 04-15-2009, 07:00 PM   #21
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Quote:
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i stand corrected. when i was installing my sri, i was informed that there was something different with the pcv valve on the '06 then the rs and it stuck with me. then again this is my first subbie and the motor is a way different beast then what i'm used to.
On your '06, the PCV valve is on the block (like on the diagram in the OP), opposed to on the intake manifold like on most older models. That's probably what you're thinking of...
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Old 04-15-2009, 07:14 PM   #22
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is all about not being able to reverse flow .
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Old 04-24-2009, 06:08 AM   #23
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Ever slight chance than when the piston nearest the PCV is heading towards dtc than the flow may reverse.... I can also bet that some bureaucrat in California would require it.
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Old 04-24-2009, 07:15 AM   #24
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Using a breather creates an opening in the system that makes your air meter incorrect - it properly measures its intake at its location, but not what actually enters the cylinder heads. The amount of difference is questionable, but it's guaranteed.
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Old 04-24-2009, 12:09 PM   #25
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It makes my car idle roughly when I disconnected the PCV hose by accident when I was working on my oil catch can.

BTW: DIY it costs about 30 dollars, and it works.
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