* Head Gasket Failure: This is a significant concern. A blown head gasket can allow combustion gases to leak into the cooling system, pressurizing it and causing overheating. The pressure can also push coolant out, leading to a low coolant level, even if you're not seeing obvious leaks. This is often accompanied by white smoke from the exhaust and/or a milky substance in the engine oil (or vice-versa).
* Radiator Issues: The radiator itself might be clogged (internal blockage restricting coolant flow), damaged (leaks or fins bent), or have a failing radiator cap (not maintaining proper pressure). Check the radiator carefully for any leaks or damage. Also, ensure the radiator cap is functioning correctly.
* Cooling Fan Problem: The cooling fan might not be engaging properly, either due to a faulty fan motor, a bad relay, or a problem with the fan's control module. The fan should come on when the engine reaches a certain temperature – even when the car is stationary. Observe if it's working correctly while the engine is hot.
* Blocked Coolant Passages: Internal blockages in the engine's coolant passages can restrict flow. This is less likely if the system was recently flushed, but it's possible.
* Incorrect Thermostat Installation: Though you replaced it, it's possible the new thermostat was installed incorrectly, preventing proper coolant flow.
* Low Coolant Level: Even if you've added coolant, there might be a leak somewhere you haven't found yet. Check all hoses, connections, and the radiator carefully for any signs of leakage, especially when the engine is hot (use caution!).
What you should do:
1. Pressure Test the Cooling System: This is the most important step. A pressure test will reveal leaks, even small ones, that you might not otherwise see. This should be done by a mechanic or using a pressure tester you can rent.
2. Inspect the Radiator: Carefully examine the radiator for any signs of damage, leaks, or blockages.
3. Check the Cooling Fan: Verify that the cooling fan turns on when the engine reaches operating temperature, even when the car is stationary.
4. Check for Exhaust in Coolant: Look for signs of a blown head gasket (white exhaust smoke, milky oil, etc.).
5. Have a Mechanic Diagnose: Given that the problem persists after replacing key components, it's strongly recommended that you take your Monte Carlo to a qualified mechanic for a proper diagnosis. Attempting further repairs without a proper diagnosis could lead to more serious damage and costly repairs.
Overheating can cause significant engine damage. Don't continue driving the car until the issue is resolved.
