Possible Causes (beyond air in the system):
* Thermostat: A stuck closed or malfunctioning thermostat will prevent coolant from circulating properly, leading to overheating. This is a very common cause.
* Radiator: The radiator itself could be clogged with debris, restricting coolant flow. Internal corrosion can also reduce its efficiency.
* Coolant passages: Similar to the radiator, internal passages in the engine block or heater core could be clogged.
* Water pump impeller: Even if the pump *appears* to work, the impeller might be damaged or worn, not moving coolant effectively. A visual inspection might not reveal subtle damage.
* Head gasket: A blown head gasket allows combustion gases to enter the cooling system, pressurizing it and potentially causing overheating. This often results in white, milky residue in the engine oil or coolant.
* Low coolant capacity: Even though you've filled the coolant, a leak somewhere in the system (hose, radiator, water pump) could be causing the coolant level to drop over time.
* Faulty coolant temperature sensor: This sensor might be providing inaccurate readings to the gauge, giving a false sense of overheating. It won't *cause* the overheating, but it could make the problem harder to diagnose.
Addressing Air in the System (if it's the problem):
If you suspect air, proper bleeding of the cooling system is essential. The procedure varies slightly by vehicle, but generally involves:
1. Finding the bleed valve (or bleeder screw): This is usually located on the engine block or a higher point in the cooling system. Consult your owner's manual for the exact location.
2. Adding coolant: Make sure the coolant reservoir is filled to the appropriate level.
3. Opening the bleed valve: Slowly open the valve to release air. You should see coolant flow and possibly some air bubbles.
4. Keeping the engine running: Keep the engine running and warm (but not overheating) while you bleed the system. Add coolant as needed.
5. Closing the bleed valve: Once coolant flows consistently without bubbles, close the bleed valve.
6. Check for leaks: Inspect all hoses and connections for leaks.
Troubleshooting Steps:
1. Check the thermostat: Start with the simplest and cheapest solution first. See if the thermostat opens when the engine reaches operating temperature.
2. Inspect the radiator and hoses: Look for obvious signs of damage, leaks, or blockages.
3. Feel the radiator hoses: While the engine is running, feel the upper and lower radiator hoses. They should both get warm, indicating coolant flow. If the upper hose is hot and the lower hose is cold, the problem is likely in the radiator or water pump.
4. Check for leaks: Inspect the entire cooling system for any signs of leaks.
5. Check the head gasket (if other checks fail): This is a more involved process and might require professional diagnosis.
If the problem persists after attempting these checks, a qualified mechanic should diagnose the issue. Overheating can cause serious engine damage.
