Originally Posted by intensifi
Warning: Thought experiment ahead. I have no scientific/engineering proof whatsoever.
It strikes me that if there are combustion by-products making their way back out to the intake valve area due to partially open intake valves during the exhaust purge that the same by-products would be sucked back into the cylinder on the next downstroke due to the vacuum created.
But if the intake valve area was "wet" with condensed oil vapor, then the combustion by-products would have something to stick to and would not ALL be sucked back in.
If true, this sounds like a perfectly vicious circle that would account for all the buildup AND support both positions of our learned co-contributors.
Also if true the solution would be to avoid the condensed oil vapors in the first place.
Lastly a chemical analysis of the oil in the catch can versus that captured from the walls of an intake port would speak volumes as to the relative contribution of each "source".
I think that analysis would certainly be definitive, but I'm not sure you're going to find someone willing to spend the time and money to collect and send the samples into someplace like Blackstone Labs to get it done. And I'm not sure what kind of sample quantity they require, so you may have problems getting adequate sample volume from the valves themselves. And lastly, I'm not sure that you wouldn't change the chemical content during the burning/coking process on the valves, which may make it impossible (or much more expensive) to match a source to the end product.
As for the rest of what you wrote about how combustion by-products sticking to the valves may increase accumulation of the PCV crud, it certainly passes the common sense test for me.
I have seen another article quoting a GM engineer stating this combustion "reversion" (as I'm calling it, right or wrong) as a source of the valve buildup, and other descriptions of manufacturers revising cam/valve timing in an attempt to mitigate the problem. But I can't deny what I saw (and photographed) in my own intake, and it was oily goo, not carbon from combustion, so I'm convinced the PCV system plays a role, too.