i had the cap removed, and then turned off the engine. Also a note to add is the coolant is surging at a rate that makes it impossible to fill while running. At the rate it is surging it would push all of the fluid out of the system if i allowed it to.
yes, this seems to have developed after the thermostat change.
I can't find any sign of intake leaking into the valley, or any coolant leak at that. Would synthetic oil make it more difficult to see coolant in oil?
I have a spare 185 thermostat, I'll change that out and see what happens