Had similar "unexplainable"
water pump">raw
water pump failures (i.e. everything clear, good impeller, good gaskets, etc etc.)
One was a failure of the screw holding the cam in place. Looked fine when opening the
pump, but saw it move when bumping the
engine.
Other was failure of the (nylon) drive
gear. Again diagnosed by bumping the
engine with the
pump cover off.