My Blazer, which I let sit for a couple of months, started overheating the first time I tried to start it. Only overheats when standing still/ idling. I have replaced the thermostat and flushed the radiator. When I flushed the radiatior I noticed water pouring from the return line of the heater core. I will probably replace the water pump anyways. But just curious about what else it could be?? I think it could be the Fan Clutch because it only over heats when Standing still. The Fan does turn but I don't think at the speed it should. Could that be it?? HELP???