* Excessive Pressure: The most likely culprit is excessive pressure building within the cooling system. This pressure forces coolant out through the overflow tube. Several things can cause this:
* Faulty Radiator Cap: The radiator cap regulates pressure. A faulty cap that doesn't release pressure properly is a very common cause. It might be failing to open at the correct pressure or sealing improperly.
* Head Gasket Failure: A blown or leaking head gasket allows combustion gases to enter the cooling system, significantly increasing pressure. This is a serious problem and requires immediate attention. Symptoms often include white smoke from the exhaust, milky oil, and overheating.
* Cooling System Blockage: A blockage in the radiator, hoses, or water pump restricts coolant flow, leading to overheating and pressure buildup. This could be due to sediment, rust, or a failing water pump.
* Faulty Thermostat: A stuck closed thermostat prevents proper coolant flow, leading to overheating and pressure buildup.
* Water Pump Failure: A failing water pump doesn't circulate coolant effectively, contributing to overheating and increased pressure.
* Incorrect Coolant Level: While less likely to cause a *leak* per se, an overfilled cooling system can lead to coolant expulsion through the overflow tube when the engine heats up and expands the coolant.
Troubleshooting Steps:
1. Check the Radiator Cap: This is the easiest and cheapest thing to check first. Replace it if it's suspect.
2. Inspect Hoses and Clamps: Look for any cracks, bulges, or loose clamps on the hoses.
3. Check the Coolant Level: Make sure it's within the normal range when the engine is cold.
4. Look for Leaks: Carefully inspect the entire cooling system for leaks, including the radiator, hoses, water pump, and heater core.
5. Check for Overheating: Monitor the engine temperature gauge while idling. If it rises significantly, you have a serious problem.
6. Pressure Test the Cooling System: A pressure test will pinpoint leaks and help determine if the system is holding pressure properly. This is best done by a mechanic.
Important Note: A head gasket failure is a serious issue that can cause significant engine damage if not addressed promptly. If you suspect a head gasket problem (white exhaust smoke, milky oil), get your car to a mechanic immediately. Ignoring it could lead to a very expensive repair.
Don't attempt major repairs yourself unless you have experience working on cars. A leaking overflow tube is a symptom; the underlying cause needs to be diagnosed and fixed. A mechanic can properly diagnose the problem and make the necessary repairs.
