* Head Gasket Failure: This is a serious possibility. A blown head gasket can allow combustion gases to enter the cooling system, pressurizing it and causing overheating. It can also lead to antifreeze leaks, sometimes in subtle ways that aren't immediately obvious (like seeping into the engine oil or leaking externally in an area not easily visible). White smoke from the exhaust and/or milky oil are strong indicators.
* Cracked Cylinder Head or Block: Similar to a head gasket issue, a crack in the cylinder head or engine block can allow coolant to escape. This might explain the leaks you're seeing.
* Clogged Cooling System: Even with a new thermostat, the radiator, hoses, or heater core could be partially blocked, restricting coolant flow. This is especially likely if you've had antifreeze leaks for some time, leading to deposits within the system.
* Water Pump Failure (despite replacement): While you've replaced it, ensure the new water pump was installed correctly. An improperly installed pump won't circulate coolant efficiently. Also, there's a (small) chance you got a faulty new pump.
* Faulty Radiator Cap: A faulty radiator cap won't properly release pressure, leading to overheating.
* Radiator Fan Issues: The radiator fan might not be turning on when it should. This could be due to a faulty fan motor, relay, or temperature sensor. This is particularly problematic at low speeds or idling.
Troubleshooting Steps:
1. Pressure Test the Cooling System: This is crucial. A pressure test will reveal leaks that might not be visible during normal operation. This helps diagnose head gasket, crack, or other subtle leaks.
2. Inspect the Entire Cooling System: Carefully examine all hoses, connections, and the water pump for leaks or damage. Look for any signs of coolant elsewhere on the engine (e.g., around the cylinder head).
3. Check the Radiator Fan: Make sure it's functioning correctly. You might need to jumper the fan relay to test the motor directly.
4. Test the Radiator Cap: Ensure the radiator cap is holding the correct pressure. A simple pressure test on the cap itself is often possible.
5. Inspect the Heater Core: If the leaks are minor and difficult to locate, a leak in the heater core could be the culprit.
6. Compression Test: A compression test on the engine cylinders can help reveal head gasket issues.
If the pressure test reveals no leaks in the obvious places, a head gasket or crack in the cylinder head/block is highly suspected and requires professional attention. These are serious repairs. Don't continue driving the car until the cause is determined, as further damage is likely.
