1. Head Gasket: This is a serious possibility. If the head gasket is blown, coolant can leak into the cylinders or the oil, causing overheating and potentially other symptoms like white smoke from the exhaust, milky oil, or loss of coolant without visible leaks. A compression test and/or a leak-down test can help diagnose a blown head gasket.
2. Radiator or Cooling System Leaks: Even with a new radiator cap, there might be a subtle leak somewhere in the radiator itself (tiny pinhole cracks are hard to spot), the hoses, or the heater core. Carefully inspect all hoses for cracks, bulges, or kinks. Look for signs of coolant leaks around the radiator, hoses, water pump, and engine block. Consider a pressure test of the cooling system to pinpoint leaks.
3. Clogged Radiator: The radiator fins might be clogged with debris, restricting airflow and hindering heat dissipation. A thorough cleaning or replacement might be necessary.
4. Faulty Fan Clutch or Electric Cooling Fan: The fan is crucial for cooling the engine, especially at low speeds or when stopped. If the fan clutch isn't engaging properly or the electric fan isn't working, the engine won't cool adequately. Check that the fan comes on when the engine reaches operating temperature. You might need to test the fan clutch's engagement or replace a faulty electric fan motor or its relay.
5. Air in the Cooling System: Air pockets in the cooling system can prevent proper coolant circulation. Make sure you properly burped the system after replacing the components. There are specific procedures for this, often involving opening a bleed valve at a high point in the system while the engine is running.
6. Low Coolant Level (despite filling): A persistent drop in coolant level even after refilling points to a leak somewhere in the system. This warrants a thorough inspection of all potential leak points.
7. Issues with the Cooling System's Thermostat Housing: While you replaced the thermostat, the housing itself could be cracked or damaged, leading to coolant leaks or air ingress. Inspect it carefully.
8. Faulty Water Pump (despite replacement): It is, albeit rare, possible that the new water pump is defective. This would require further investigation.
What to do next:
1. Pressure test the cooling system: This is a critical step. A pressure test will reveal even small leaks that visual inspection might miss. You can rent a pressure tester from most auto parts stores.
2. Check for exhaust gases in the coolant: This helps confirm or rule out a blown head gasket. Use a coolant testing kit available at auto parts stores.
3. Thoroughly inspect all hoses and connections: Look for cracks, bulges, or signs of leakage.
4. Check the fan's operation: Ensure it's turning on when it should.
If you're not comfortable performing these checks yourself, take your car to a qualified mechanic for a proper diagnosis. Overheating can cause significant engine damage, so it's crucial to address the problem promptly.
