Hmmm the pumps mechanical isn't it? Meaning it just has a impeller spinning around a housing, right?. It seems they usually seize up or leak, right? Can the Impeller fall off internally (no flow). If it siezes it would burn or throw the belt right? What about the thermostat sticking shut. That can cause over heating pretty quick. Not sure if it will allow you to run it with the radiatior cap off? If so, watch the rad fluid, it should be still, then when the thermostat opens the fluid should start flowing, no flow, might be not opening. You can also remove the thermostat, put it in a pot, heat on stove and check it with a thermometer. If its a 210 it should open at 210. Also could possibly be a blown head gasket, cracked head or block. Any water in oil, or oil in the water? Oh, plugged radiator? Rad shop can pressure test it.
