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When the car is off and cold, the overflow reservoir shows a lower reading versus when the car is on and warmed up.

This is confusing because when the car is cold, it can be said that coolant needs replenishing as it is closer to the minimum mark, but after the car is warmed up, it can be said that it does not need replenishing, as it is closer to the maximum mark, so which one is right?

What state should the car be in when reading of the overflow reservoir, to make sure it is at the optimum level?

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usually the coolant res should be between MAX and MIN mark when the engine is completely cold, but this varies by manufacture, so check your owners manual. Coolant expands when it gets hot, so when your engine is warm, the overflow tank will contain more fluid. whereas when its cold the coolant is more dense, thus lowering the coolant level.

  • While the temperature increase, so does the pressure and, if you check, the cubical expansion coefficient for liquids is very small... – Solar Mike Oct 24 '17 at 8:56
  • very good point – Pnelego Oct 24 '17 at 18:25

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