I have a 1992 F150 with a 302. I just recently replaced the water pump and thermostat due to overheating issues. The water pump was defiantly broken. Since then I have run the truck for 1 week exactly. Today the truck ran hot for the first time but the only place water could be found was boiling out of coolant reservoir. When I replaced the water pump and thermostat I flushed the coolant system. Does anyone have any ideas what the issue could be? Also my heating system works perfectly fine.

 

Defective thermostat or installed backwards preventing it to fully open. Could also be a bubble block rare but has happened.

 

What is the part number of the waterpump you used? Is it correct rotation? How hot did it run previous to replacing waterpump? Could be a hurt head gasket.

 

I'd go with btexan's advice on checking the thermostat first. The big sensor part goes towards the motor (to measure it's temperature) and the pointy end goes towards the rad.
Even installed correctly, brand new thermostat's have been known to fail. Especially cheap ones. Go top of the line - your thermostat protects your motor from overheating, which cooks your motor. Last thing you want to face is having to buy a new motor over a few bucks difference in thermostat price.