Coolant leak only when parked

Symptoms

• Puddle appears under the car after parking, not noticed while driving
• No warning lights or overheating while driving
• Puddle size may vary depending on how long the car sits
• Coolant level still drops gradually over time

Likely causes

• CommonLeak from a low hose or fitting that only drips once flow stopsSome leaks are small enough that active circulation and airflow while driving don't show a drip, but it becomes visible once the car is stationary.
• CommonRadiator cap releasing slight pressure as the system coolsAs coolant cools and contracts after shutdown, a marginal cap seal can allow a small amount to weep out.
• PossibleWater pump weep hole leaking only under certain conditionsSome pump leaks are more apparent when the engine isn't running and pressure settles.
• PossibleReservoir or hose leak that only shows once coolant is at restA crack that seeps rather than sprays may only be noticeable once the vehicle is parked for a while.

What to do

1. Place clean cardboard under the car in your usual parking spot to catch and locate exactly where drips form.
2. Check the radiator cap's seal condition, since this is a common source of parked-only weeping.
3. Check the coolant level over a week or two to confirm it's actually decreasing, not just normal minor fluctuation.
4. Avoid ignoring a small but consistent leak, since even slow leaks add up and can eventually cause low-coolant problems.
5. Have a mechanic pressure-test the system, ideally leaving it under pressure for a while to reveal slow leaks that don't show during a quick test.
6. Replace the radiator cap as an inexpensive first step if its seal looks at all worn.
7. Get any confirmed leak source repaired rather than just monitoring it indefinitely.

Tools and parts

• Cardboard or leak mat

When to call a pro

See a mechanic for an extended pressure test if the leak is elusive, since slow parked-only leaks can be harder to catch during a quick shop visit.

Safety