I know what your getting at regards the bmw pcv systems , smoking the manifolds doesnt always show external leaks , a sign that there is excessive vaccum getting into the breather system is the oil filler cap gets sucked on and you can feel the vaccum as you try to remove it .

If you can remove the vaccum source from the breather system and blank it ,then see if your manifold vaccum goes up to about 350 mb

HTH

Lee