Thermostat and water pump are both good
guesses. For what it costs I would replace the
thermostat and gasket without hesitation, if its
horizontally mounted and has a small jiggly pin
make sure the pin is UP as it lets trapped air out. To
get a quick idea if the water pump is any good once
the truck is up to temperature squeeze the upper
rad hose, the pressure should be pretty firm, if not it
may be weak. Make sure all air is purged from the
cooling system, if you park on a small upwards
incline it will speed up the process. Did you remove
the radiator fan shroud? It's required to be there for
proper cooling. Check the radiator for clogged or
bent fins, without enough airflow going through the
fins you won't have proper cooling. Check to make
sure the lower rad hose hasn't been replaced with
the wrong one, it must be smaller in diameter that
the upper hose. Other than any of that your radiator
may have become internally clogged, A CLR
(internal) soak and a back flush can clear out MOST
radiators. That's all i got for now
6 people found this helpful.