1. Head Gasket Failure: This is a significant possibility. A blown head gasket allows combustion gases to enter the cooling system, pressurizing it and forcing coolant into the overflow. Symptoms often include:
* White smoke from the exhaust: Look closely for white, milky exhaust. This indicates coolant is burning in the cylinders.
* Coolant in the engine oil: Check your oil dipstick. Milky or frothy oil is a strong indicator of a head gasket problem.
* Loss of compression: A compression test can reveal if combustion gases are escaping into the cooling system.
2. Cracked Cylinder Head or Engine Block: A crack in the head or block can also cause coolant to leak into the system under pressure. This is less likely than a head gasket failure but still possible.
3. Warped Cylinder Head: Overheating can warp the cylinder head, causing sealing issues. This often leads to a head gasket problem.
4. Incorrect Thermostat Installation: Although you've replaced it, double-check that the thermostat is installed correctly and opens at the correct temperature. A faulty or improperly installed thermostat can cause overheating.
5. Radiator Cap Issue: A faulty radiator cap might not be releasing pressure correctly, leading to excess pressure in the system and forcing coolant into the overflow.
6. Water Pump Issues (despite replacement): While unlikely if you just replaced it, there's a small chance the new pump is faulty or not installed correctly.
7. Cooling System Leaks (other than the pump): Inspect all hoses, clamps, and the radiator carefully for leaks, especially when the engine is running. A small leak can lead to overheating and pressure buildup.
Troubleshooting Steps:
1. Pressure Test the Cooling System: This is crucial. A pressure test will reveal if there are leaks in the system that aren't visible.
2. Check for Exhaust Smoke and Oil Contamination: Look for the telltale signs of a head gasket problem (white smoke, milky oil).
3. Verify Thermostat Operation: Observe the thermostat to ensure it's opening and closing as expected. You can do this by testing it in a pot of boiling water.
4. Re-inspect the Water Pump Installation: Make sure the pump is properly seated and the impeller spins freely.
5. Inspect all hoses and connections: Look for any leaks, cracks, or loose clamps.
6. Replace the Radiator Cap: This is a cheap and easy fix that might solve the problem.
Important Note: If you suspect a head gasket or cracked cylinder head, do not continue to drive the vehicle. Driving with a damaged head gasket can cause catastrophic engine damage.
It sounds like you may need a professional mechanic's help to diagnose the problem accurately, especially if you suspect a head gasket issue. A pressure test and possibly further diagnostic tests are necessary.
