It helps to remember how the PCV operation is set up. The PCV offers a way for oil to disappear without any obvious signs of lack of compression or leaks.
The valve is a one-way system to prevent backfires popping the engine. Normally it will allow the inlet manifold depression to scavenge the crankcase fumes, and a bit of clean fresh air is fed in to the other side of the engine from before the throttle body. There is always oil vapour in the crankcase and cam cover, but the amount scavenged is partly a function of how much blow-by there is; full throttle on a well worn engine will help heat the oil for more vapour and the increased blow-by at hot throttle will push more oil vapour through the valve with less of the fresh air.
It is not a bad thing because a little will help with the inlet valve guides lube and a tiny bit of upper cylinder lube before the oil burns off.
Too much oil vapour will produce blue smoke!
Too much blow-by will produce emulsion under the oil-filler cap.
Both a stuck open or stuck shut PCV will have a detrimental effect, because there are other systems in place expecting to balance up its effects on the air and exhaust supplies to the inlet manifold.
A healthy one will be clean inside (not black and crusty) and the ball inside will rattle when the valve is shaken.
The small diameter breather pipe from the rocker cover to the large air intake pipe is clean and dry inside, as is the underside of the oil filler cap. How do I identify the PCV valve to check the ball is free, please? I assume it is somewhere along the large rubber pipe that connects the air filter to the throttle body? Bear in mind my mechanical expertise is largely based on the BMC “A” series engines fitted to my Morris Minor and assorted Mini’s . . . .
The PCV valve on the NC is under the intake manifold and requires removal of said manifold to access.
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Being as the insides of the oil filler cap, and the breather pipe from the rocker cover to the air intake are spotless, can I assume that the PCV valve is working?
That is a very good start. It suggests the ventilation is normal.
If there was any serious blow-by it would be likely to contaminate that tube as well as the emulsion build-up in the cam box cover. This also suggests the rings and bores are OK.
PCV was just a possible thought, and now the search continues elsewhere!
Your clean plugs tend to rule out oil from the inlet valve guides and EGR as well, unless they were cut after a period of gentle running.
If the exhaust is dry and not sooty when running at normal temperatures then the chances are that no significant quantity of oil is being burnt. If it is sticky black that is a sign of oil (I walked away from four such NCs when looking for my MX5.)
The inside of the exhaust pipes is black, with a black, powdery rather than sticky deposit on the outside at the bottom, looking like it might have been washed out when moisture is expelled on startup? II guess there’s some burnt oil?