Unless Kip-on-truckin has changed his opinion, this is from his thread "blew another engine" post 6 , 03-24-2017
Good ideas fellas. It was me who postulated that this problem, which happened to me and to another guy on the board, was caused by excessive idling. I think we can rule that out. This engine, installed 170k ago in Feb of 2016, got idled a little bit at the tail of winter 16, but I didn't idle it at all this winter. Didn't need to, figured out other solutions to keep warm. It was just a wild guess anyway - so far as I've been able to find out there's no connection between idling and carbon, or idling and valve issues. As a pure highway van, I idle FAR less than the average soccer mom idling in the drive thru lane of a Starbucks anyway.
Nope, my guess is that when I get the head off I'll find that the valve seat has fallen out of the head and is loose on it's seat, just like the last engine. My mechanic partially ruled out a lifter or rocker issue because those would have constant symptoms. My misfire comes and goes, and is highly load dependent. And the vacume loss means it has to be intake valve related.
My mechanic did put a horoscope on it and didn't see anything noteworthy except two shiny spots on the top of the piston. This could be normal, or it could mean that it was drinking coolant. But the van wasn't loosing a drop of coolant, nor is it burning a drop of oil. When I have the head off, I can compare it to the other cylinders. It may turn out that the problem is something I haven't thought of, but since it's identical to the previous problem, and the pentastar has a history of valve seat issues (supposedly resolved by 2013) I have no choice but to assume that's the problem.
One remote possibility is the problem is magnified by the internal EGR design. The pentastar, like a CAT Acert, overlaps the valve timing to keep some exhaust gas in the chamber. This is why it doesn't have an EGR valve - it does it internally through cam timing. It's possible that if the cam wears, or the cam phasers get funky, it draws in too much exhaust and that carbons it or otherwise fouls it. Dunno. I'm clutching at straws.