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Does Coolant Level Drop While Engine Is Hot? (Explained)

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Last Updated on: 7th September 2023, 12:38 am

A common question that a car owners ask is whether the coolant level drops when the engine is hot.

The coolant level can actually rise slightly while the engine is hot, this is due to thermal expansion. As the coolant fluid heats up it expands, and some of the coolant can be pushed into the reservoir tank.

However, if the coolant level rises too much it can overflow and lose coolant fluid which is why it’s important not to overfill the coolant reservoir.

In this article, we’ll discuss if and why coolant level drops while the engine is hot.

a car coolant reservoir tank

Does Coolant Evaporate When Hot?

Another reason that could lead to an increase or decrease in coolant level is evaporation. Given that the coolant within the system is subjected to elevated temperatures, it may result in the evaporation of some water from the cooling system.

However, most vehicles cooling systems are sealed and pressurised and don’t allow evaporated gases to escape.

Is It Normal for Coolant Level to Drop While Engine Is Hot?

No, the coolant level shouldn’t drop while the engine is hot because most cooling systems are sealed and pressurised, it’s normal for the coolant level to rise slightly when the engine is hot due to thermal expansion.

If the coolant level lowers significantly, it could be a sign of a malfunction in the cooling system.

What Should the Level Be?

The optimal coolant level is between the “min” and “max” marks on the reservoir tank. The level should be between these two marks, regardless of whether the engine is hot or cold.

The coolant level should be at or above the “min” mark when the engine is cold.

Overfilling the coolant system can cause it to overflow and be expelled through the pressure relief valve and expansion tank, leading to a loss of coolant fluid.


To summarise, the coolant level can rise while the engine is hot due to thermal expansion, if the coolant level lowers significantly, it is important to have it checked.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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