I've always thought the minimum and maximum mark on the coolant bottle were quite pointless as it doesn't seem to make much of a difference to the function of the vehicle if I overfill a little bit or under fill, I really only used it as a rough guideline to show me if I'm low on coolant need to top off. Recently though I've had to deal with a few coolant leaks which made me curious.

My understanding is that liquid cooling systems in cars operate under pressure to increase the boiling point of the liquid (glycol) which allows it to absorb more heat from the engine and dissipate it to the environment making the system overall much more efficient. I suspect the engineers have designed system to have a certain amount of air inside of it as air compresses unlike liquids and therefore the system needs a little bit of "cushion" to stop hoses and pipes from blowing up (or collapse due to vacuum), but not too much "cushion" as then the system wouldn't be able to reach correct operating pressure.

Also it seems that the same bottles are used by many car makers across the industry for all kinds of models so I'm guessing there's very few important variables that determine required air-to-liquid ratio. (The ubiquitous VW bottle https://i.stack.imgur.com/rXie6.jpg)

Is my suspicion somewhere in the right ballpark or am I dreaming?

  • I can tell you the VW bottle is far from "ubiquitous". It is used within the VAG, but beyond that I've never seen the round bouncy ball anywhere else. I've always kind of assumed it was their trademark reservoir. May 14, 2023 at 0:23

1 Answer 1


There are a couple of reasons: firstly overfilling can be a problem as the air above the coolant has to take up excess pressure as the engine warms up. Secondly, having min and max quite close together gives an early warning of a leak developing. As you say though, running a little below min isn’t generally a problem.

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