Diagnosing Overheating and Cooling Concerns

Check coolant level first. Low coolant is the most common cause of overheating. If coolant is full, feel the upper radiator hose with the engine at operating temperature. If the upper hose is cold — the thermostat is stuck closed and coolant is not circulating to the radiator. Replace the thermostat.

The vehicle runs fine at highway speed but overheats in traffic or at idle. The cooling fan is not working. With the engine at operating temperature, the fan should be running. Check the fan relay. Check the fan motor by applying direct battery power. Check the coolant temperature sensor signal that triggers the fan activation. A fan that does not turn on at operating temp is the most common cause of overheat-at-idle complaints.

The coolant level keeps dropping but you cannot find a puddle under the vehicle. Internal head gasket leak. Combustion gases are pushing into the cooling system, or coolant is leaking into the combustion chamber and being burned. Perform a chemical block test at the radiator neck with the engine running and cap off — if the test fluid changes color, combustion gases are present in the coolant. Check the oil for a milky tan color indicating coolant mixing. Check the exhaust for persistent white sweet-smelling smoke after the engine is fully warmed up.

Feel both heater hoses at the firewall with the engine at operating temperature. Both should be hot. Both cool — the thermostat is stuck open, the engine is not reaching operating temperature. One hot, one cool or cold — the heater core is restricted internally and coolant is not flowing through it. Both hot but no heat from vents — the blend door actuator that directs air across the heater core is stuck in the cold position. Scan the HVAC module for blend door codes.