Most Likely Culprits:
* Water Pump: A failing water pump won't circulate coolant effectively, leading to overheating. You should be able to feel the upper radiator hose getting hot when the engine's running, and the lower hose should also be warm (though not as hot as the upper). If the lower hose is significantly cooler than the upper, the water pump is suspect.
* Thermostat (despite being new): While you've replaced it, it's possible the new thermostat is faulty or stuck closed. A faulty thermostat prevents coolant from circulating properly until the engine reaches dangerously high temperatures.
* Head Gasket: This is a serious problem. A cracked head or blown head gasket allows combustion gases to enter the cooling system, pressurizing it. This pressure can force coolant out, even without visible steam from the exhaust (especially at lower pressures). This is why checking for a cracked head is vital.
* Radiator Cap: A faulty radiator cap prevents the system from properly building and maintaining pressure. Low pressure means the coolant boils at a lower temperature, leading to overheating.
* Cooling Fan: Make sure the cooling fan is working correctly and coming on at the appropriate temperature.
How to Check for a Cracked Head or Blown Head Gasket:
There are several ways to check for this, ranging from simple to more involved:
1. Pressure Test the Cooling System: This is the most reliable method for detecting a leak in the head gasket or a crack in the head. A mechanic uses a pressure tester to pressurize the cooling system and look for leaks.
2. Exhaust Gas Test: A combustion gas tester checks the coolant for the presence of exhaust gases (like hydrocarbons). If exhaust gases are present in the coolant, it's a strong indication of a head gasket failure. This test is more definitive than a visual inspection or pressure test.
3. Compression Test: A compression test measures the compression in each cylinder. Low compression in one or more cylinders can indicate a head gasket leak or a warped head.
4. Visual Inspection (limited value): While unlikely to provide conclusive evidence without removing the cylinder head, check for signs of coolant leaks around the cylinder head itself.
What to Do:
1. Check the obvious: Verify the cooling fan operation, feel the radiator hoses, and examine the radiator cap. Ensure the lower hose gets warm.
2. Pressure test the cooling system: This should be your next step. A mechanic can perform this relatively quickly.
3. Consider the exhaust gas test: If the pressure test is inconclusive, this test will provide a more definitive answer regarding head gasket integrity.
Important Note: Driving a car with an overheating engine can cause serious and irreversible damage. Do not continue to drive the vehicle until the cause of the overheating is diagnosed and repaired. Have it towed to a mechanic if necessary. Ignoring the problem could lead to costly repairs or engine failure.
