Test the water pump: One way to test for a bad water pump is to squeeze the upper radiator hose when the engine is hot and idling. Careful, because teh hose willb e HOT! If you do not feel much coolant circulating through the hose when you rev the engine, the pump may be bad. The other cause might be a bad thermostat that is not opening properly (remove and inspect the thermostat), or a clogged radiator.
Could be airlock in the system try bleeding it don't work get it block and pressure tested.