* Water Pump: A failing water pump won't circulate coolant effectively. This is a very common cause of overheating. If the pump is weak or failing, the coolant might not be moving properly through the engine, even with a new thermostat. The heater core acts as a bypass when the heater is on, improving circulation slightly.
* Radiator: A clogged radiator or a problem with the radiator cap (not allowing proper pressure) can prevent proper heat dissipation. Low coolant level, even if you haven't noticed a leak, could indicate a slow leak or a problem with the radiator's ability to hold pressure.
* Cooling Fan: The cooling fan might not be turning on when it should, or it could be malfunctioning. This is especially crucial at low speeds or when idling. A faulty fan relay or a bad fan motor itself could be the culprit.
* Head Gasket: A blown head gasket is a serious issue. Coolant could be leaking into the combustion chamber or into the oil. This can cause overheating, especially if the engine is pressurizing the coolant system. This is less likely if the heater trick works, but it's still possible.
Why the heater helps (temporarily):
When you turn on the heater, you're forcing coolant through the heater core, which improves coolant circulation. This temporarily relieves the pressure and helps cool the engine. However, it's a temporary fix and doesn't address the underlying problem.
What to do:
1. Check Coolant Level: Ensure the coolant level is correct and that there are no visible leaks. Top it off if necessary, but don't open the radiator cap while the engine is hot.
2. Check Radiator: Inspect the radiator for clogs or damage. You can carefully feel the radiator hoses to see if they're getting hot.
3. Inspect Water Pump: Look for leaks around the water pump. You might be able to hear a bearing whine if it's failing. This often requires a mechanic to diagnose reliably.
4. Test Cooling Fan: With the engine running, check if the cooling fan comes on. It should turn on when the engine reaches a certain temperature. If not, investigate the fan's electrical components (relay, wiring, fan motor itself).
5. Pressure Test the Cooling System: A mechanic can perform a pressure test to check for leaks in the system.
6. Consider the Head Gasket (If other issues are ruled out): If the other possibilities are eliminated, a head gasket problem should be investigated. This is a much more complex and costly repair.
Do not continue to drive the vehicle until the cause of the overheating is determined and fixed. Driving it while overheating can cause serious engine damage. It's best to have a mechanic diagnose and repair the issue.
