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I’m a little concerned that there is a far stronger mix of antifreeze in my car's coolant system than I am aware. I check the antifreeze level in my car (using a refractometer) when the engine is cold (after being left overnight).

During the night, can the antifreeze settle to the bottom of the cooling system, thus leaving a weak mixture of water and antifreeze within the coolant reservoir up top, where I take a sample with the refractometer?

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    $\begingroup$ No. The only way for any separation to occur would be if some of the coolant froze. The frozen part would be rich in water, the liquid part rich in ethylene glycol. $\endgroup$ – MaxW Dec 12 '15 at 21:57
  • $\begingroup$ Ethylene glycol and water are miscible in all proportions: en.wikipedia.org/wiki/Ethylene_glycol. That said, if you were to add antifreeze directly to the coolant overflow bottle, as is normally recommended, it would take a few restarts and cool-downs of the engine before it mixed with that in circulation. $\endgroup$ – DrMoishe Pippik Dec 13 '15 at 2:16
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Ethylene glycol is completely miscible with water in all proportions (as the commenters point out). Therefore, once mixed, the glycol will not separate from the water, ever.

The only exception is if it gets so cold the mixture starts to freeze. Freezing will cause some degree of separation as the water will solidify first. But if it is that cold, separation will be the least of your problems.

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