It is a one way pressure valve. I guess my other engine is letting water back flow and shouldn't which completely threw me off when that one would backflush. My gut says this was never about anything
stuck in the system, I suspect all along its been a bad thermostat. Gotta love boats.....

mercury has a device lie that on some engines. it is normally upright front of engine, just before t-stat housing. best to pull hoses and work thru. is hose disconnected from pump.