This is going to seem like a silly question, but exactly how does the PCV Valve work?? - I understand that it's to scavenge emissions from the crank case so you have the other hose that allows filtered air INTO the rocker cover, then there's the PCV Valve and it's hose that draws air from the rocker cover and into the manifold - that's WHAT it does - I'm after HOW it does it.

I assume it sucks closed under high vacuum (ie. at idle) and opens / allows air to be drawn thru to the manifold at lower partial vacuum conditions?? - ie. from a certain level of open throttle onward?

Okay - did what I should have done originally and searched the net - the PCV does work as I suspected - ie. under idle (high) vacuum the valve is sucked to a "closed" condition which still enables some flow but it's fairly seriously restricted - then as vacuum reduces thru part throttle condition, it opens up and more thru flow is enabled.

My point in raising all this is that to be blunt I suspect those of us with aftermarket cams might be suffering from the PCV valve not being sucked properly closed due to the reduced idle vacuum that our cams make. This may be contributing to the nasty / fluctuating / cold stalling idle characteristics that some people are experiencing.....

I think that I may also be suffering an intermittent smoke blowing problem as a result of my idle vacuum being very borderline for being able to close the PCV valve properly (as well as occasional idle raggedness) - tho it may be just the elderly condition of my PCV valve - ie. the smoking has stopped and idle has been consistently very good after giving my PCV valve a damned good shake. Maybe just a new one is in order?

I'd love to try a valve that sucks closed somewhat easier than the std factory Falcon one - buggered if I know how to locate a suitable one tho - I guess a trip to several wreckers and some experimentation is in order.

PCV probably doesnt help with high duration cams but the problem is more to do with the I6 measuring load based on manifold pressure. Case in point being the V8 engines with air mass used to calculate load/VE - the efi windsors can take quite a radical cam and the ECU cops it pretty well. The manifold pressure still drops much like the I6 but as EEC is basing load on the mass of air entering the engine - it deals with it pretty well.

Exactly - ie. if the PCV valve isn't closing quite properly it's going to be exacerbating the idle manifold pressure problem - ie. in addition to a cam that isn't producing a heck of a lot of vacuum you have in effect an additional manifold leak - and yes, all this is in the context of Speed Density system of the I6 - "6 Cylinder Workshop"

Im not sure about the I6 but the V8 has a closed PCV system. i.e. there are two lines to the throttle body/manifold. One has the pcv valve from the throttle body to the manifold (effectively to the crankcase) the other is the 'vent' line from the crankcase (connected to the oil filler neck) back to the throttle body... so any air drawn from the crankcase through the PCV valve is replaced by air drawn from the throttle body... for the V8 this means that no unmetered air is entering the system and the load calcs are correct... im not sure what the implications are for a syetm based on manifold pressure??

Yeah, on a MAF system, as long as any takeoffs are downstream of the MAF sensor and that air is ultimately put back into the intake system - ie. into the inlet manifold (the PCV valve line on the V8 probably is into the TB on the engine side of the butterfly? - it needs to be exposed to manifold vacuum) - then the ECU is able to calculate according to total airflow into the engine....

But on the I6 with it's Speed Density system then the PCV system is essentially simply a manifold leak. Evidently the correctly functioning PCV system "leak" is taken into account as a known quantity in the ECU "calibration" - where I'm coming from is that in engines with mods that lower the idle vacuum, perhaps the amount of leakage might be greater than the system is "calibrated" to have? (ie. because of the reduced idle vacuum not pulling the PCV valve properly "closed").

This is a good read fellas. Not sure if I unserstand it all but seeing that my cam is quite lumpy...tell me how you go green machine.

No worries - I'll update with anything I come up with. Do keep in mind tho that I'm just conjecturing on all this - it may well be that my line of thought here is completely incorrect - perhaps the PCV valve shuts at half the vacuum that even a very lumpy cammed engine has at idle so simply isn't an issue unless it's sick.

Next thing I really need to do is get a new PCV valve and see how that goes as perhaps what I'm seeing and what's leading me down this whole line of thought is simply due to my old PCV valve being rooted - but the next thing I'm wondering is whether a PCV valve that does close completely - ie. no flow at all at idle - might still be a good mod for those of us with chunky cams so we can get as much vacuum as possible...

hmmm - it might even come down to using a solenoid and vacuum switch - or a microswitch off the throttle (ie. so the PCV line is closed at idle but allowed to operate normally from a little off idle).

You have the theory backwards, at idle the vacuum actually opens the PCV valve allowing the crankcase gasses to recirculate back into the manifold, when you hit the throttle the vacuum drops therefore the valve closes (although any pressure in the sump from blowby will open the valve again). Removing the PCV system may give you a slight increase in idle vacuum and may help with idle but I don't think it would be all that noticable. It will also make the car fail emissions testing.

I did a net search and found a couple of sites that describe the PCV Valve operation as I have - ie. it's pulled "closed" under idle vacuum - but it isn't really completely closed as there's still a calibrated amount of flow thru the valve when it's internal mechanism is in the full closed position. This makes sense as you don't want a big induction leak at idle.

Then as the vacuum drops off it opens more fully and reaches it's highest flow position at intermediate throttle position - and then as vacuum continues to drop off even further toward full throttle then it goes to a closed position again but such that case pressure can bypass around it easily. This also provides backfire protection - ie. PRESSURE from the manifold will snap it closed - but any pressure from the case will unseat it and flow thru.

In a broad sense the PCV valve as I understand it operates like a one way check valve with flow direction from the case to the manifold - so of course manifold vacuum is going to pull flow thru it - but it's then designed to restrict the flow at idle - ie. high vacuum. Also I suppose high pressure from the case end will also drive it to that restricted position - but you'd have a real sick engine to do that!

And there lies the point of my conjecture - ie. what happens if at idle there's not enough vacuum to be pulling it properly to the idle restricted position??

Just get a valve and have a look at it and blow through it (get a new one because the fuel and oil deposits aren't good for you LOL), and you will clearly see that vacuum on the engine side of the valve opens it while no vacuum allows the valve to close up.

At idle and light cruise while there is sufficient manifold vacuum the vacuum pulls the built up gasses from the crankcase, thats how the PCV system works. When there is no vacuum (at WOT) it does act as a vent too because of pressure in the sump.

The whole reason I raised this topic is that I used to hold that same MISCONCEPTION - then when I realised that the valve is indeed NOT supposed to be fully open at idle I started thinking about a) what might cause that; and b) what the resultant effect might be.

The possible answers I came up with to those two questions are: a) insufficient vacuum to properly close the PCV valve at idle - ie. due to a big cam maybe?; and b) the result would be a manifold leak - which would exacerbate the already low vacuum situation - causing crappy idle.

Here's a quote from an American site - tho it's oversimplified and doesn't clearly explain that the maximum flow will be at intermediate vacuum conditions and that at WOT the valve is actually opened by case pressure - it does illustrate what I'm trying to say about the situation at idle:
"At idle the engine isn't working very hard and the valve remains mostly closed, allowing a small amount of the crankcase gases back through the engine. At full throttle the engine works harder and creates more crankcase gases, so the PCV valve opens wide to let more through. If the PCV valve is stuck open at engine idle, too great a quantity of spent gases are let through. Rough engine idling and stalling can be the result."

From what I can see, the I6 PVC valve is basically a ball in a vertical tube - probably with grooves in the top seat to enable the calibrated flow at max vacuum and a full sealing seat at the bottom so that if there's reverse flow (backfire) it'll snap completely closed - but case POSITIVE pressure can still flow by. So at idle the vacuum flow is great enough that the ball is sucked up the tube and held into the top seat with a certain amount of "leakage" around it - then as the vacuum reduces and is insufficient to hold the ball against that top seat then it'll just sort of "float" in the body of the tube - in this state there'll be the maximum amount of flow occurring around it and thru the valve overall - and then at WOT with virtually no vacuum the ball will sit on the bottom seat where it'll still let positive case pressure thru.

Edit/addition: have also found references on American car sites dealing with modded engines - ie. hotrods - that when using a cam that drops idle vacuum from original std specs then care needs to be taken to ensure a suitable PCV valve is also used or else the valve will not be closing properly at idle and possibly causing rough running etc.

Ok then that explains it a bit better, and yeah I see that even though it is open at idle it still is designed to provide some restriction to maintain vacuum.

As for your idea to modify the system to try and increase manifold vacuum and help idle, I did try it on my engine with very little result. My V8 runs only 8" of vacuum at idle, an I6 would never even idle with that low vacuum, I would imagine the large camed I6's would be in the mid teens. With the PCV system totally blocked I only saw a 0.5-1" gain, if that would be enough to help idle at all I doubt it but it may be worth a try.

Yikes

What cam did you put in the poor thing grant?? My stock HO runs a solid 19"...

Excellent info on the result with completely blocked PCV Valve - as you say, it's not much - but in a marginal situation it might make a difference on the 6 cylinder and show more of a result (ie. with the speed density system).