Quote Originally Posted by NoWake View Post
My first thought is the heater core. How was it bypassed? Almost sounds like could be leak in one of the hoses(supply or return). Warm to hot water while running makes me think this.
This bypass was done by the previous owner or his/her mechanic. It appears they've taken a cut out of the longer section of hose that feeds the heater core and simply looped it from the hot water output on the side of the block back into the return on the bottom of the water pump.