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PCV Valves

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I have been trying to fit a PCV valve between an inlet manifold into an existing camshaft breather vent.

 

But the PCV valves I have bought don’t work as I thought they were supposed to. I believed that at Idle the valve would close, or almost close to prevent the high vacuum weakening the mixture and ruining the idle. As the revs build and the vacuum drops the valve would progressively open until the valve was fully open and there is no, or very little restriction from the valve. That is what I wanted.

However the valves I have don’t work that way. Taking either (of three different types)  valve and at either end, if I suck the valve moves towards my mouth and provides a 100% seal. If I then blow the valve centre then repositions and provides a 100% seal . A gentle suck or blow never finds a sweet spot where the valves passes much air, or in my case wind! If fitted to the engine they block flow at all times.

 

Obviously I have the wrong PCV valves, but what do I actually need to provide a flow between perhaps 5% to 100% proportionate to a variable vacuum?

 

Alan

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Feel your pain.  I have a suspicion that many modern ones actually sense flow rather than pressure and rely on the engine being quite well sealed, whereas the older ones (like the Smiths "Flying Saucer" type) do work on pressure and draw a small amount of diluting air through the engine via a small metering orifice usually in the oil cap.

When messing about with my PI breather system I eventually found an AC branded valve originally intended for a GM V8.  P/No was  CV770C.  There is a whole family of these with slightly different operating pressures but I can't find the table listing their details that I found before.  Not sure the one I bought was my first choice pressure-wise but it was available!  Came from US ebay and very reasonably priced except for the damn shipping.

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