Sand and dirt have a way of following gravity, to my thinking, you could have the block sorta plugged up on the lower side with sand and grit. Would not necessarily affect flow through the o/b dump. What you're gonna have in that situation is less water spending less time absorbing heat in the engine, and maybe not in the right places. Think physics.
The engine is still producing the same amount of heat, no matter what, but because maybe sand or whatever has settled in the bottom of the water passages, also heating up with the block, that same heat is being applied to a subsantially less volume of water. Temp goes up. I'd be worried, because water temp is just an indicator that something is wrong, meaning if something like this is happening, something else in the engine is getter FAR hotter than it should.
I'd do the hose thing, while on the trailer, just to make sure there's flow and not a simple problem. Then yeah, as Glenn said, pop out a plug or two and see what comes out or what it looks like in there (I'd do the rearmost ones, the hardest ones to get to). A byatch, yes, but much cheaper than a new engine.
I'd also probably at home hook a hose to the outlet side (Vise grip the header hose), unhook the inlet side and sorta backflush it to see what comes out.
Just another .02 observation.
For future reference, I dunno if everyone, or anyone, does this, but I do. I pull the block drain plugs out right after I trailer the boat(s) home (hafta pull the starter on the bitch side). I don't use pet cocks, just thread tape the plugs again and put them back in. Petcocks will act as a sort of filter, but maybe I'm being anal about it, then I'll run the hose hookup through the motor with them uncorked for a few minutes. Just me.
Ray
