* Overpressure: This is the most common reason. The cooling system is pressurized to help prevent boiling. If the pressure becomes too high, it will force coolant out of the overflow reservoir. This could be caused by:
* Faulty radiator cap: A malfunctioning radiator cap can fail to regulate pressure correctly, allowing it to build up excessively.
* Head gasket leak: A blown or leaking head gasket allows combustion gases to enter the cooling system, increasing pressure. This is a serious issue. You'll often see white smoke from the exhaust, loss of coolant, and potentially overheating.
* Cracked cylinder head or block: Similar to a head gasket leak, a crack in the engine itself can allow pressure to build in the cooling system.
* Clogged radiator: A restricted radiator can't dissipate heat effectively, leading to increased pressure.
* Water pump failure: A failing water pump can cause overheating and subsequent pressure buildup.
* Thermostat stuck closed: A stuck closed thermostat prevents coolant from circulating properly, leading to overheating and pressure build-up.
* Low coolant level (less likely if it's *coming out*): While a low coolant level wouldn't *force* coolant out, if the level is extremely low and the engine overheats, the expansion of the coolant can create enough pressure to force some out. This often indicates a leak somewhere in the system.
Important Note: Driving with a coolant leak can lead to serious engine damage from overheating. Do not continue to drive the car until the issue is diagnosed and repaired.
To diagnose the problem, you'll need a mechanic to inspect the cooling system thoroughly. They'll likely check the radiator cap, pressure test the system, inspect the head gasket (possibly requiring a pressure test specific for this), check the hoses, and assess the radiator and water pump. Ignoring this could result in catastrophic engine failure.
