If it's overheating at idle or slow speed it's an airflow
problem. Check the radiator cooling fans and be
sure the front of the radiator is not blocked by dirt,
leaves, or other debris. If it overheats at expressway
speeds it's a coolant circulation problem. Water
pump, thermostat, blocked cooling system
passages, air trapped in the cooling system, and
possibly a collapsed lower radiator hose. Rev the
engine and see if the suction from the water pump
is pulling the lower radiator hose shut. Since the
water pump and thermostat are new I'd suspect a
clogged radiator or clogged cooling system
passages in the engine if you're overheating at
expressway speeds. Changing the heater core will
not help. Are you getting heat? If not that points to
air in the cooling system or a clogged heater core. If
the heater core is clogged you can bet the rest of
the cooling system is too! Hope that helps! Jim
3 people found this helpful.