End tank blew off?
Did a good flush and fill/basic service and it started overheating the next day. Bunch of crap came loose from somewhere I think is some kind of "head gasket in a bottle" type stuff, or at least heavy duty stop leak. Had to've been years old as I don't use that sorta thing. Was full of fibers, seems kinda like cotton but don't know exactly what. Enough to completely clog the little hose that goes from radiator to overflow tank so you know the radiator core is clogged too.
Took it all apart and took it down to the carwash, pressure sprayed it out til I got good clean flow both directions through all the little slots.
I'll consider it truly fixed when/if I get a few drive cycles/couple hundred miles on it without any trouble but for now it holds pressure, doesn't leak, and doesn't overheat.
Probably replace it eventually anyway just for peace of mind but figured why not give it a shot. If it doesn't work I'm only out $10.
I suppose I shouldn't be surprised if I find a head gasket leak now as they must've put that shit in there for a reason.