* Weak or failing radiator cap: The pressure cap is crucial for maintaining system pressure. If it's weak or faulty, it won't hold pressure, and coolant might seep out when the pressure drops after the engine cools down. This is a relatively inexpensive and easy fix.
* Cracked or damaged hose: Hoses, especially those that get hot and cold repeatedly, can develop cracks or weaknesses over time. These might only leak when the pressure is reduced after the engine shuts off. Inspect all hoses carefully for cracks, bulges, or soft spots.
* Leaking radiator: A crack in the radiator itself is another possibility. This is usually more noticeable as a consistent leak, but a small crack might only leak when the system is depressurized.
* Water pump weep hole: While uncommon to leak *only* when off, a failing water pump can sometimes seep coolant from a weep hole. This typically indicates the pump seal is failing and the pump will need replacing soon.
* Head gasket leak: This is a more serious issue. A head gasket leak might not show significant symptoms when the engine is running due to the pressure, but the pressure drop when the engine is off could cause a leak to become apparent. This often presents with other symptoms like white smoke from the exhaust or loss of coolant without any visible leaks.
* Block crack: Less common, but a crack in the engine block could also cause a leak that's more noticeable when the system is not under pressure.
How to diagnose:
1. Visual inspection: Carefully examine all components of the cooling system, including the radiator, hoses, connections, water pump, and the pressure cap itself. Look for any signs of coolant residue, wetness, or damage.
2. Pressure test: A mechanic can use a pressure tester to pressurize the cooling system and pinpoint the location of the leak more accurately. This is the most reliable method.
3. Check for other symptoms: Look for other signs of potential problems such as white smoke from the exhaust, overheating, milky oil, or low oil levels (all potentially pointing to a head gasket leak).
It's recommended to have a mechanic diagnose the leak properly. Ignoring a coolant leak can lead to engine overheating and significant damage. The relatively simple fixes (hose, cap) are cheap, while the more serious ones (head gasket, block crack) are expensive and require professional attention.
