Stupid PCV question...

[kcryan]

Dont ask why i want to know this...but

Is there ever a case when the PCV system could help the engine to draw more fresh air into the intake? not recirculated gasses, but fresh air?

Thank you, and please excuse my randomness.

 

[Quest]

a vacuum/PCV hose leak may have caused this. This is fairly common on neglicted systems with bad vacuum actuators/hoses, cracked PCV hoses, etc.

 

[kcryan]

Good point, in a fully functioning system, is it possible?

 

[Quest]

depending on the degree of blowby in your piston rings, motor oil "boiloff" rate,etc. typically, the amount of fresh air coming from a functional closed PCV system (from the crankcase and re-introduced into your intake side) maybe 30% (not 100% sure) and it's already been factored in into your computer controlled system already.

Unless something catastrophic happens to your PCV system, otherwise, there is no way (it's a metered system done using intake vacuum) you can have a lot of unmetered fresh air being introduced into your closed-loop system.

Care to elaborate on your situation?

 

[kcryan]

Just a debate between me and another guy, he says that PCV systems help bring in fresh air, i say they dont, they relieve pressure from the crankcase and put it into the intake tract but have no real effect on fresh air intake, if anything they lower it because they put air in themselves, therefore decreasing the need (and amount brought in) of fresh air, right?

Thanks

 

[ikeepmychevytoo]

Does PCV rely on the blowby? Some percentage of it is fresh air from the begininig, it is a ventilation system not a closed system, if I don't miss some basic thing.

There must be an intake to the crankcase from the air cleaner housing (or a separate deviced filtered intake) and a ported vacuum for suction (in many situations). On wot, both equalizes to a degree and suck the blowby gases, on others, when there is little blowby (as in idle) it just draws fresh air, circulates it through the crankcase and to be sucked in the port.

 

[johnd]

These days the crankcase is a closed system with no connection to the outside world beside the PCV valve which is connected to the intake and lower pressure there. Assuming there are no ways for air to get into the crankcase there will never be anything sent to the PCV valve that doesn't consttute blowby. That being said there are lots of places where the outside air _could_ be leaking into the crankcase. Lots of gaskets, seals, etc., but in a properly functioning system, no.

 

[1sttruck]

In addition to the PCV valve the 3.0 V6 on the Taurus also has a tube from oil filler neck to the air intake, which evidently acts like another PCV valve when needed, or can allow air for the PCV system as the vacuum at the PCV is typically higher.

Between the oil filler tube, the PCV valve and the EGR system the intake gets pretty gunky loooking.

 

[V1]

What's the deal with that extra PCV you speak of 1sttruck? Same thing as the PCV on the rear without the valve?

 

[mechtech]

The vast majority of PCV systems have a filtered inlet.
Of course they ingest fresh [ambient] air - and some is piped to the manifold along with the blowby and possibly oil.
Under hard acceleration, you may get blowby out of BOTH the inlet and PCV valve. The blowby coming out of the inlet has no fresh air at this time, and is admitting none.

 

[Gary Allan]

quote:

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Originally posted by kcryan:
Just a debate between me and another guy, he says that PCV systems help bring in fresh air, i say they dont, they relieve pressure from the crankcase and put it into the intake tract but have no real effect on fresh air intake, if anything they lower it because they put air in themselves, therefore decreasing the need (and amount brought in) of fresh air, right?

Thanks

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I think what you're asking is "does the PCV system represent a significant source of combustible air??"

The PCV valve is a regulated manifold leak. It doesn't care what it sucks in. If there is very little blow by .."air" will be what it is sucking. In higher CFM situations, after adjusting for the length and diameter of the hose ..and subsequent upsteam (to the atmosphere) restrictions ..it is treated NO DIFFERENT then the throttle body opening. It will move a % of the total air in proportion to how it "looks" to the available vacuum and represent a (albeit variable) % of the total CFM.

It is "factored in". Now not all systems have PCV valves. Some, like my two jeeps and my Mitsubishi 3.0 use metered orifices. This doesn't work in the same manner. It will extract a certain amount of volume and then the excess is sent upstream of the throttle body. Either way all the blow by emissions get to take a trip through the combustion chamber.

 

[LouDawg]

quote:

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Originally posted by 1sttruck:
In addition to the PCV valve the 3.0 V6 on the Taurus also has a tube from oil filler neck to the air intake, which evidently acts like another PCV valve when needed, or can allow air for the PCV system as the vacuum at the PCV is typically higher.

Between the oil filler tube, the PCV valve and the EGR system the intake gets pretty gunky loooking.

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Ditto on my 2.4L Nissan with the oil filler neck air return...

 

[1sttruck]

"What's the deal with that extra PCV you speak of 1sttruck? Same thing as the PCV on the rear without the valve? "

It's not a PCV valvle, it's just a hose connecting the oil filler neck to the air intake, but at high output it can suck air from the crankcase like the system with the PCV valve does.

 

[Gary Allan]

I had some arrangement like that on my 19 ....hmmm..(I forget) Ford Fiesta with the Cortina enigne (the high performance "ghia" version with 2bbl weber no less

), I believe. The manual actually showed how the flow initially flowed into the normal location via the PCV ..then, at some point (higher speed), was drawn via the other tube.