Yes, coolant can absolutely leak from your car even when the engine is off. This is a common issue, and it's often easier to spot when the car is stationary because the leak isn't being blown around by the wind or burned off on hot engine parts. The root cause is usually a failure in the cooling system's ability to hold pressure.
The cooling system is a closed, pressurized loop. When the engine runs and heats up, the coolant expands, increasing the pressure inside the system. This pressure is regulated by the radiator cap. Even after you turn the engine off, the system often remains pressurized as it slowly cools down. A weak point—like a cracked hose, a failing water pump seal, a damaged radiator, or a faulty radiator cap—will allow coolant to escape under this residual pressure.
Here are some common leak points and their characteristics:
| Common Coolant Leak Source | Typical Symptoms/Indications | Approximate Repair Cost Range (Parts & Labor) |
|---|
| Radiator Hose | Puddles under the front/mid engine; visible cracks or bulges on hoses. | $150 - $400 |
| Water Pump | Leak from the front of the engine (often behind pulley); whining noise when running. | $500 - $1,000 |
| Radiator Cap | System doesn't hold pressure; may lead to overheating; leak from the cap seal. | $20 - $80 |
| Radiator Itself | Puddles directly under the radiator; visible corrosion or damage to fins. | $500 - $900 |
| Heater Core Hoses | Puddles under the dashboard area (passenger side); sweet smell inside cabin. | $300 - $800 |
| Thermostat Housing | Leak from where the thermostat is installed (usually top of engine). | $200 - $500 |
| Head Gasket (Severe) | Coolant mixing with oil (creating a milky sludge on oil cap); white smoke from exhaust. | $1,500 - $2,500+ |
If you suspect a coolant leak, the first step is to check the coolant level in the overflow reservoir when the engine is cold. Never open the radiator cap on a hot engine. To pinpoint the leak, you can place a large piece of cardboard under the car overnight. The location of the drip on the cardboard will help you identify the general area of the leak. Driving with a significant coolant leak can lead to engine overheating and severe, costly damage, so it's important to address it promptly.
