Vanagon T3 Wbx PCV tower valve operation

There’s been much speculation as to the intent and actual function of the Vanagon Waterboxer engine’s Positive Crankcase Ventilation (PCV) tower and its internal control valve, often called in VW and Bentley the “crankcase breather”. It’s also an open question as to how much effect or benefit a functioning valve has on oil usage, since so many of these have failed internally yet it apparently makes little or no difference.
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Broadly, the logic isn’t mysterious: An internal rubber diaphragm valve closes when the engine air intake tract is at some heretofore unknown pressure lower than atmospheric, aka vacuum or underpressure. Closing the valve creates a deliberate restriction in the path for crankcase gases to flow to the intake, otherwise that path is large enough to be unrestrictive, the gases are free to flow into the intake at the rate they are being produced so crankcase pressure will theoretically not increase above Manifold Absolute Pressure (MAP).
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The place it ports into and where the underpressure that would act to close the valve exists is the intake air rubber S-boot, after the Air Flow Meter (AFM) and before the throttle valve. When the throttle is closed, as at idle or overrun, there would be only atmospheric pressure in the S-boot. When the throttle tips in, at first there would be underpressure to some degree and up to some combination of throttle angle and engine load, and as the throttle is opened even further that pressure would rise again to almost atmospheric.
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VW’s literature claims that the PCV control valve is closed at idle and high engine vacuum. In fact only the latter is true, since at idle the intake manifold after the throttle is under high vacuum but the boot before the throttle is at atmospheric pressure. But even so, at idle there’s no need to restrict PCV volume because there is so little, rpms are very low, the throttle is restricting intake air volume making in-cylinder compression and combustion pressure low, little fuel is being burned, so there is relatively little blow-by volume. Also with the throttle closed gases can only flow thru the idle air circuit, which is itself a tight restriction.
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On the flipside, at high load the intake tract is basically at atmospheric pressure, so unfortunately when blow-by gas volume is greatest, there is really nothing to draw it out of the case into the intake, the gas pressure in the case will have to rise above MAP to push the gases there. That’s the essential and insurmountable problem with PCV in general as concerns blow-by gases, it’s only “positive” when it’s not needed and passive when it is most needed. But apart from blow-by, the crankcase is always a maelstrom of oil mist, so limiting PCV flow under high intake vacuum should at least have the benefit of limiting oil consumption by that route. Should.
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So it’s supposed to restrict crankcase ventilation volume at low-to-mid load cruising conditions, and open wide to allow unrestricted ventilation in high-load operation when the rate of blow-by is the greatest.
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All of these aspects can be deduced just by looking at the elements of the system, the only question that remains is exactly how much underpressure makes the valve close, in other words what did they choose as the intake manifold vacuum transition between low and high load?
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Not that hard to find out by hacking one to watch it work. Luckily I saved a few good ones from my former engine building operations. Take a look.
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The test rig: The top of an unmolested PCV tower appears to be a sealed cap, directly below which is the cloth/rubber diaphragm that constitutes the moving valve element, which is about the same diameter as the tower itself; by being so large it takes only slight pressure to deflect it. But for the diaphragm to move at all, outside air must be able to flow in and out of the closed space above the diaphragm, so it’s well known that under the cap rim there is a very tiny vent to the area above the diaphragm. Since that’s functionally no different than having nothing covering the diaphragm at all, I cut the cap off a tower and cut out the center so I could directly watch the diaphragm movement. The crossed wires and clamp are simply holding the cap down firmly to seal the diaphragm to the rim of the tower body, helped by some sealant.

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The tower bottom is blocked and airtight, and the breather outlet is plumbed to an electric vacuum pump as a steady source of suction. The vacuum gauge is teed into the suction hose, with another tee where I can vary the opening with my fingertip to let air in and control the amount of suction.

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Note that the vacuum gauge’s bottom stop is one inch of mercury, not zero. So you can see the transition level is in the range of 1.5 to 2.5″ Hg., by which the diaphragm is drawn down flat against the seat. It will seal more and more firmly under increasing underpressure.

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2.5-3″ Hg. is the level I have long set as the transition pressure for my control device that forces the ECU into a rich high-load mixture, so this agrees with my own experience as to what MAP is at high load. Since I have a dash LED to show me the state of my switching device, I know that at low to mid-throttle opening and load the intake vacuum is more than 3″Hg., so knowing that ~2.5″Hg. will close the PCV valve, I now know it will be wide open at high engine load and/or large throttle openings, closed at every other load condition, and wide open at idle.
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I could install this hacked tower and a video camera to watch it in actual driving, but with this test and knowing my mixture device behavior I now have a pretty complete picture of the conditions under which the valve is limiting crankcase gases (and some oil mist) into the intake system.

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