* Faulty radiator cap: This is the most common cause. The radiator cap regulates pressure within the cooling system. If it's faulty (worn out, letting pressure escape too easily, or failing to release pressure when it's supposed to), coolant will be forced into the overflow tank when the system pressurizes, but it won't properly return to the radiator when the engine cools down. The pressure should be consistent in the system as the engine warms. A bad cap can cause the whole system to run low on coolant.
* Head gasket leak: A blown or leaking head gasket can allow combustion gases to enter the cooling system. This increases pressure, forcing coolant into the overflow. You might also see white smoke from the exhaust, loss of compression, overheating, milky oil (oil contaminated with coolant), or sweet-smelling coolant. This is a serious problem requiring immediate attention.
* Cracked cylinder head or block: Similar to a head gasket leak, cracks in the engine block or cylinder head can allow combustion gases into the coolant, building pressure. This usually involves significant overheating and other symptoms like the ones mentioned with head gasket failure.
* Water pump: A faulty water pump might not circulate coolant effectively. While this is less likely to directly cause the *pressure* issue, poor circulation could contribute to overheating and indirectly lead to pressure buildup, forcing coolant into the overflow.
* Thermostat: A stuck closed thermostat will prevent coolant from circulating properly, leading to overheating and pressure buildup. A stuck *open* thermostat might cause the engine to take too long to warm up, but is less likely to cause this problem.
* Clogged radiator: A severely clogged radiator restricts coolant flow, causing overheating and pressure buildup.
How to diagnose:
1. Start with the radiator cap: Replace it with a new cap of the correct pressure rating for your vehicle. This is the cheapest and easiest fix first.
2. Pressure test the cooling system: A radiator pressure tester will help identify leaks in the system (radiator, hoses, etc.) and show if pressure builds up excessively. This test is crucial to ruling out head gasket or block problems.
3. Inspect for other symptoms: Look for evidence of overheating (steam, hot spots on the engine, etc.), white exhaust smoke, milky oil, sweet-smelling coolant, or any external leaks.
4. Check the thermostat: Ensure the thermostat opens and closes properly as the engine warms and cools.
Important Note: If you suspect a head gasket leak or crack in the engine block/head, do *not* continue driving the vehicle. This can cause severe engine damage.
Before attempting any repairs yourself, especially if you suspect more serious problems, it's best to consult a mechanic. A proper diagnosis is essential to prevent further damage to your engine.
