Operation of PCV valves...

[betyouaint]

I've been reading the PCV Education thread and from what I understand, there are 2 PCV valves on the 2.0T; the one that sits above the intake manifold and the other (sometimes referred to as the "rear PCV" or breather tube) that sits above the turbo. Can anyone give a description of how these valves operate together. As the updated PCV (from N to P) requires both parts be replaced, I'm assuming that their action is complementary...? What would be the result of updating the main PCV but not updating the rear PCV? Also, it seems the main PCV is more prone to failure but does that mean the rear PCV rarely fails? What is the expected life of the main valve?

I've also heard that the valve cover plays a role and it too can "malfunction". What role does it play and what causes it to malfunction?

[Jay-Bee]

Well this is all answered in the thread you refer to but to put it simple.

The valve cover is baffled, and has "separation chambers" i guess you could call them to route pressure out the 2 available passages.

The 'main' PCV on the intake manifold side has a one way check valve, vacuum from the intake manifold draws this oil/vapor/air into the engine, when it sees boost it closes off.

and then...

The 'rear' comes into play, it is really just a conventional style valve cover vent tube, as it is always seeing some airflow across the opening and pulling it's vapors down the turbo and intake tract. Some models had a check valve, when the main pcv is blocked off this would make noise and needed to be removed.

There is also an oil separator in the oil filter housing and that is the second tube you see come up as another source of relieving lower crank case pressure

[CorneliusRox]

So... If you ignore the oil separator and the passages in the valve cover and everything and you only think of it as crank case pressure, it feels simpler.

The Rear PCV has a check valve only letting pressure to leave the crack case, the Main PCV also has a check valve only allowing pressure to leave the crank case.
The way these two differ is that the rear one is before the turbo so it is always in vacuum, and the main one is past the turbo so it gets boost sometimes.
The boosted part is the failure prone part because once you enter boost, the check valve blocks air/boost from rushing into the crank case. When they fail, it is really just the check valve failing and allowing boost to run into the crank case. This throws oil everywhere.

To understand the rear PCV, you need to realize that an engine pulses and the crank case venting is a pulse. Without the rear PCV it will just pulse back and forth (which isnt a bad thing), but with the check valve it will pulse to push air out, and then when it pulses to pull some back in, it can't and it creates a vacuum. The vacuum does two things really. The first is it is supposed to help you piston rings seat better allowing less blowby and more in cylinder pressure giving you more power (I'm not convinced this actually happens, or it is just like .01hp), the second is now that there is vacuum you can potentially pull more blowby in if your piston rings arent perfect (this is why I removed it in my system).

There is nothing wrong with plugging the front PCV and using only the rear PCV, but you will get oil/blowby going through your turbo, intercoolers, and intake manifold. This does nothing to the turbo, but collects oil in the intercoolers (killing efficiency), and leaves carbon on your intake valve causing misfires (this would happen even with the front PCV).


Hope that helps.

[Brillo]

CorneliusRox
I'm trying to better understand the PCV system on our cars too as I've been considering a catch can for some time, mainly to reduce carbon on the intake valves. Isn't it true that regardless of which PCV (main or rear) is in play at the moment the that blow-by gases and oil mist will makes it way into the induction system? Therefore, for those using catch cans, ideally wouldn't you want both the main and rear PCV gases to travel through the catch can prior to entering their respective ports at either the turbo or induction system? Another question: the front PCV enters the induction system after the throttle body, right?

[Mister W]

There is nothing wrong with plugging the front PCV and using only the rear PCV, but you will get oil/blowby going through your turbo, intercoolers, and intake manifold. This does nothing to the turbo, but collects oil in the intercoolers (killing efficiency), and leaves carbon on your intake valve causing misfires (this would happen even with the front PCV).

So best thing would be to block front pcv to intake and put a CC on the rear pcv tube? You say "leaves carbon on your intake valves" how could it do that if you block the front pcv by removing the tube that goes to the intake?

[betyouaint]

Thanks guys, the mists of ignorance are clearing a little and it's starting to make sense. Every time I lift the hood of this car I see another little gizmo with a hose or two attached and possibly some electrical connection but often have no idea what the purpose of said gizmo is. My previous AWD turbo car seems like child's play compared to the Audi. The problem I have is that I need to truly understand something before it even starts to make sense. I can't get by on half sort of knowing how it works...

[CorneliusRox]

CorneliusRox
I'm trying to better understand the PCV system on our cars too as I've been considering a catch can for some time, mainly to reduce carbon on the intake valves. Isn't it true that regardless of which PCV (main or rear) is in play at the moment the that blow-by gases and oil mist will makes it way into the induction system? Therefore, for those using catch cans, ideally wouldn't you want both the main and rear PCV gases to travel through the catch can prior to entering their respective ports at either the turbo or induction system? Another question: the front PCV enters the induction system after the throttle body, right?

Yes and Yes.
I think the best way is to block the front PCV with plugs, then block the turbo inlet of the rear PCV and use a venturi into the exhaust so that the rear pcv outlet off of the valve cover has vacuum to the exhaust after the cat. Keep a check valve in place too if you want.

So best thing would be to block front pcv to intake and put a CC on the rear pcv tube? You say "leaves carbon on your intake valves" how could it do that if you block the front pcv by removing the tube that goes to the intake?

Because the vapor will still enter the same system as if it did through the front PCV, just now it has to go through the turbo, intercoolers, and throttle body. It is better because the vapor will be caught on the walls of everything, but wont fully stop it. And then your intercoolers wont operate nearly as well as they used to. An easy fix to that is to drain them once every few months if you like. It is easy and only takes a flat head screwdriver.

Thanks guys, the mists of ignorance are clearing a little and it's starting to make sense. Every time I lift the hood of this car I see another little gizmo with a hose or two attached and possibly some electrical connection but often have no idea what the purpose of said gizmo is. My previous AWD turbo car seems like child's play compared to the Audi. The problem I have is that I need to truly understand something before it even starts to make sense. I can't get by on half sort of knowing how it works...

Honestly, I think that this care is INSANELY easy to work on. What was your old AWD car? The CR-V? The most I have worked on those is swapping out O2 sensors and spark plugs.

[betyouaint]

Honestly, I think that this care is INSANELY easy to work on. What was your old AWD car? The CR-V? The most I have worked on those is swapping out O2 sensors and spark plugs.

Haha... no... although the CR-V really is the easiest car in the world to work on. My previous car was a Vauxhall (aka Opel, aka Holden) Calibra Turbo 4x4 which wasn't available in North America. The standard 204hp was easily pushed to 285 and over 300 with a few extra mods. I once clocked myself at 163mph using GPS and even have a photo of the speedo at 155 mph. For many years the Calibra held the record for the lowest drag factor of any production vehicle and also had a 6 speed manual box when most other cars were just getting a 5th gear to play with. I had mine for over 10 years and I knew it like the back of my hand... and yes, I miss it dearly. I had to sell up 5 years ago when I moved the US from the UK.

[CorneliusRox]

Very cool. I've never heard of this.

Haha... no... although the CR-V really is the easiest car in the world to work on. My previous car was a Vauxhall (aka Opel, aka Holden) Calibra Turbo 4x4 which wasn't available in North America. The standard 204hp was easily pushed to 285 and over 300 with a few extra mods. I once clocked myself at 163mph using GPS and even have a photo of the speedo at 155 mph. For many years the Calibra held the record for the lowest drag factor of any production vehicle and also had a 6 speed manual box when most other cars were just getting a 5th gear to play with. I had mine for over 10 years and I knew it like the back of my hand... and yes, I miss it dearly. I had to sell up 5 years ago when I moved the US from the UK.

[Charles.waite]

Haha... no... although the CR-V really is the easiest car in the world to work on. My previous car was a Vauxhall (aka Opel, aka Holden) Calibra Turbo 4x4 which wasn't available in North America. The standard 204hp was easily pushed to 285 and over 300 with a few extra mods. I once clocked myself at 163mph using GPS and even have a photo of the speedo at 155 mph. For many years the Calibra held the record for the lowest drag factor of any production vehicle and also had a 6 speed manual box when most other cars were just getting a 5th gear to play with. I had mine for over 10 years and I knew it like the back of my hand... and yes, I miss it dearly. I had to sell up 5 years ago when I moved the US from the UK.

AKA the street version of this:




FAP FAP FAP!

[betyouaint]

AKA the street version of this:




FAP FAP FAP!

Yeah... only mine was quicker...

Only just realized, my old website has been offline for a while as I forgot to renew the domain...

http://www.legoheads.com/micksmix.co...libraturbo.htm

[SwiftA4]

A lot of good information listed in the first couple of posts that has helped me better understand our PCV system. Also very good information in the "PCV education" thread. I've read through some of it to try and get some questions answered, just didnt seem like my answers were there.

As others have posted recently, I'm beginning to get the "chugging" sound which everyone has kindly stated is the "rear PCV" check valve. I'm stock using the oem pcv, obviously no catch can setup. It happens only on a cold idle and once the car warms up or i start to drive away, it disappers.

From reading all of the information posted about the PCV, my understanding is that the "Main PCV" is operating at idle, using the vaccum created by the intake manifold, to pull vapors out of the crank. The "Rear PCV" should then be closed. When in boost, the "main" check valve closes and doesn't allow boost into the crank. If the check valve is faulty, boost pressure will enter the crank causing possible symptoms of leaking valve cover gasket, oil cap, and all the other symptoms everyone has kindly listed.

The "Rear PCV" is operating under boost, producing a vaccum to pull the vapors out of the crank/vc. When at idle, the "rear" check valve should be closed as the "main PCV" is venting the vapors.

My questions lies in "Why is the rear check valve failing?"... I haven't seen anyone answer that question, i've just seen responses say "It fails, and you can just remove it.". Does it just essentially "fail" in general terms? Is the chugging sound at cold idle produced by the check valve being stuck open, since it should be closed under idle conditions?

Is it caused by a bad "main PCV"? I'm not blowing oil everywhere, I did have a slight leak from the vc but Audi replaced it and i havent seen the leak since, i do not have any idle fluctuations. I've tried to get Audi to replace the PCV system, but with no MIL they won't touch it.

Is there a bad side to removing the check valve? In all honesty, i'm probably just going to change the "rear PCV" tube out anyway, instead of removing the check valve.