What's the REAL Functionality of the PCV VALVE?

[vristang]

I understand what the pcv SYSTEM does and how it works. I have posted my pcv/oil in intake FAQ here several times.
My question is why is the pcv VALVE needed?

I always assumed it was to prevent the crankcase from exploding in the event of an intake manifold backfire. But if you go on any Boost forum, those guys have a rough time keeping boost out of the crankcase. So if the pcv does not prevent boost from entering the crankcase, how would it stop a flame front if a back fire should occur?

Also, every time I have inspected a pcv for directional flow, it has failed horribly. Since ALL of the NEW pcv valves I have checked did not limit backflow, I have always just installed them anyway. Never gave it much thought until now.

Any thoughts would be welcome

jason

 

[wythors]

The PCV is simply an emmissions device that allows unburned gases to recirculate and be used again in the combustion process. If you just Google "What does the PCV valve do" you will find over a hundred entries that explain this in detail. If you're getting oil in your intake, you have sealing issues in your heads allowing too much oil past your valves or excessively high pressures inside the engine for some reason.

 

[wild86stang]

An EGR valve recirculates exhaust gases, not a PCV valve. I had the same assumption as you, vristang, that it was in place in case of backfires. Hopefully somebody else will shed some light on this.

 

[blackcloud50]

In short, during the normal combustion process, there is always some air/fuel charge that escapes past the rings into the crankcase. The pcv valve is in place to ventilate or evacuate the crankcase of this unwanted pressure and for emissions purpose to reburn the air/fuel charge

 

[89Notch]

It is suppose to keep boost from entering the crank case, I don't know how well it is designed though

 

[blackcloud50]

It is suppose to keep boost from entering the crank case, I don't know how well it is designed though

Tha's not what the real functionality of pcv valve is

 

[vristang]

I was afraid I would get this kind of response.

Like I said I know what the pcv system does, venting crankcase pressure to the intake manifold. Venting pressure from the crankcase in a n/a motor does not require a directional valve though. So, why is there a valve in the system?

The same question stands.

Why is there a diractional valve in the pcv system? The pcv has been proven not to control flow from the intake to the crankcase, so what is the point of the valve?
Could we all just trash the valve and run a straight hose to the intake?

jason

 

[vristang]

The PCV is simply an emmissions device that allows unburned gases to recirculate and be used again in the combustion process. If you just Google "What does the PCV valve do" you will find over a hundred entries that explain this in detail. If you're getting oil in your intake, you have sealing issues in your heads allowing too much oil past your valves or excessively high pressures inside the engine for some reason.

Like 86stang said I think you have the pcv confused with the egr system.

Here is what I have posted on the pcv, and controlling the oil flow out of the crankcase.
I guess I should add some theory on how the pcv works.

The first step is to find out where the oil is coming from.
Pull the Throttle Body/Valve Cover hose from the TB. If it is wet with oil then this is an entry point. The normal causes of this are excess blowby (which needs to be ruled out with a compression test), or the VC baffle has been removed for rocker clearance. Just looking down the oil filler neck should reveal if the baffle is in place. If you see rockers, then it is time to get creative and fab a baffle. If there is no baffle but the TB/VC hose is bone dry then you may be able to get away without using a baffle.

Next pull the pcv hose. Again if it is wet with oil, this is an entry route. The best solution I have seen for this is a separator from Home Depot or Lowes. For about $25 for the separator and fittings you will be able to remove most of the oil before it gets to the intake.
Also, you need to verify that the pcv screen is in place. This often forgotten part is located underneath the pcv in the back of the intake, and helps remove oil mist from the crankcase gasses.

From what I have seen, the oil accumulation in the intake will be greatest when the car is left to idle for some time (stuck in traffic or idling in the driveway), then with some rpm the oil is picked up and thrown into the combustion chamber.

This is a pic I took of my engine at its worst. The car had been at idle for several minutes before I shut it off.
http://forums.stangnet.com/attachmen...8&d=1123979745

Hope this helps some,

Reference:
Home Depot
This is the Fram pcv screen pn #BA8113
This is the NAPA pcv screen pn #6978

 

[wythors]

An EGR valve recirculates exhaust gases, not a PCV valve. I had the same assumption as you, vristang, that it was in place in case of backfires. Hopefully somebody else will shed some light on this.

I didn't say "exhaust gases" genius, I said "unburned gases" which are the byproducts of gas, oil and spark in a closed environment.

Since nobody seems to be willing to read anything other than this forum, here's what autochannel.com has to say about the PCV valve:

The positive crankcase ventilation valve is a simple device that allows crankcase pollutants (water, acids, etc.) to be sucked into the intake system where they're burned. It usually consists of a tube with a spring-loaded "bullet" that interrupts a hose from the manifold to the crankcase. It opens under intake vacuum and allows the fumes to be sucked in. Intake manifold backfires can't set off the fumes in the crankcase since the "bullet" only operates in one direction. It's important that the PCV valve is clean and free-acting.

 

[vristang]

I didn't say "exhaust gases" genius, I said "unburned gases" which are the byproducts of gas, oil and spark in a closed environment.

Since nobody seems to be willing to read anything other than this forum, here's what autochannel.com has to say about the PCV valve:

The positive crankcase ventilation valve is a simple device that allows crankcase pollutants (water, acids, etc.) to be sucked into the intake system where they're burned. It usually consists of a tube with a spring-loaded "bullet" that interrupts a hose from the manifold to the crankcase. It opens under intake vacuum and allows the fumes to be sucked in. Intake manifold backfires can't set off the fumes in the crankcase since the "bullet" only operates in one direction. It's important that the PCV valve is clean and free-acting.

No need to get nasty. Misunderstandings happen.

The confusion came from your use of a few terms that are usually reserved for speaking about egr. "Unburned gasses" is not the term normally used in regard to pcv; "Blowby" or "combustion byproducts" would have made more sense to the reader. Also the term "recirculation" is not entirely appropriate here. It's not a question of right and wrong, just what most people use in day to day conversation. I was a little confused by your use of terminology myself.

I have done plenty of reading outside this forum. I studied Automotive Design in college. I have some knowledge of what the pcv is supposed to do.
As your qoute states, it is supposed to contain backfires.

My point is why do we think it will contain a backfire, when it cannot contain the boost pressure from a turbo or supercharger?
Are all pcv valves useless in reality?

 

[89Notch]

Tha's not what the real functionality of pcv valve is

Do you have a point? I was answering one of his questions.