Found the issue. There was a piece of metal jammed in the thermostat. This piece of metal was in the shape of a roughly hewn washer, about 2 1/2 inches in diameter. It flowed through the system and tried to flow through the thermostat when it was open. It forced the thermostat to remain in the open position constantly. I have not been able to figure out where, or from what, this foreign object originated. It looks like someone tried to hand fashion a rather large washer out of a freeze plug, but that's just a wild guess.
Thanks for your input.
Darin