This week, a Toyota Prius driver filled up the car’s coolant tank in the dark. Since they couldn’t see the “min” or “max” marks clearly, they measured with their heart. Knowing it might not be at the correct level, they left the vehicle to sit until daybreak. Sure enough, when they went to check the next day, it was overfilled by more than an inch.

The driver spent time internally pondering whether or not the overfilled tank was safe to drive with. They Googled the idea, which garnered “scary” results. Maybe he should just drain the tank and start over…

Not wanting to oversweat the issue, they went to r/AskAMechanic

Their question garnered a decent response, with mixed answers. The most upvoted reply?

“You can overfill coolant to the brim, car won’t care. There’s a hose on the tank that just points at the ground, if it’s overfull it’ll just spit out what it doesn’t want.”

The statement earned 83 upvotes.

But overfilling a car’s coolant reservoir can lead to potential risks, even though modern cooling systems are designed to expel excess coolant through an overflow tube or reservoir. Here’s what you need to know:

What happens when you overfill the coolant?

Coolant expansion and overflow: As the engine heats up, the coolant expands. If the reservoir is overfilled, the expanding coolant will be forced out through the overflow tube. This may seem harmless, but it can create messy leaks and, in some cases, spill onto hot engine components.

Increased system pressure: While cooling systems are designed to handle pressure fluctuations, excessive coolant could put additional strain on hoses and seals. If the excess coolant isn’t easily expelled, it might cause premature wear or even failures in gaskets, hoses, or the radiator.

Airflow blockage and cooling inefficiency: In extreme cases, excessive coolant could cause fluid to be forced back into the system in a way that disrupts normal flow, leading to reduced cooling efficiency. This might contribute to overheating rather than preventing it.

Environmental and mechanical concerns: Spilled coolant is not only toxic to pets and wildlife but can also cause a slippery mess on engine components, potentially leading to belt slippage or corrosion if left unchecked.

Is it mechanically safe to drive with too much coolant?

Short-term: Most likely, yes, as long as the system properly expels the excess.

Long-term: It depends. If the overflow system works as designed, the main risk is minor coolant loss. However, if excess pressure builds up or hoses and seals are weakened, it could lead to failures down the road.

What should you do?

If you’ve overfilled the coolant slightly, the system will probably self-correct by expelling the excess.

If it’s significantly overfilled, it’s best to drain the excess to avoid unnecessary strain on the system.