I'd run a small diameter coil spring down the blocked hose, or maybe a flat plumbing snake, or hit it with some air pressure. Is it possible you are deadheading against the thermostat? Try removing it and hitting the hose with hose water again. The FSM is spotty on which hose is which. The clamped hose should be a direct flow off the water pump. The flanged hose connection looks like it comes off the lower intake, but I can't remember the exact water routing. Been too long since I've been in there.

Any body got an upper intake off and could shoot some pics? I'm pretty sure the flanged connector is the return side, but not sure enough to make any definative comment.