Last Updated: 30.03.2022

If you wake up to an empty coolant reservoir, you might assume it evaporated.

So, does engine coolant evaporate?

The truth is…

The coolant doesn’t evaporate because it’s in a somewhat closed system. But, some faults in the system can cause a drastic drop in coolant levels beyond the normal evaporation that happens with any fluid. There could be external and internal leaks in the cooling system.

Before we look at solutions, let’s find out:

Why Your Car Is Losing Coolant

Possible causes:

A Faulty Reservoir Cap

The radiator cap maintains reservoir pressure so that the coolant travels through the hose from the engine to the radiator. 

A faulty cap causes the coolant to evaporate or reduces reservoir pressure, and you’ll see a warning light when that happens.

man holding a reservoir cap

Sometimes, the usual loop for the coolant changes because the reservoir cap is rusty or worn out. Other signs that your reservoir cap is faulty are an overheating engine, white strips on the radiator due to coolant leakage, ruptured radiator hoses, and reservoir overflow.

Head Gasket

It’s an essential component in an engine as it prevents the coolant from mixing with the oil as they loop through the engine to the cylinder head.

Damage to the head gasket can cause unexpected coolant level drops. It gets worse when the coolant seeps into the combustion chamber. For example, if your engine releases white smoke, the coolant may be burning off in the combustion chamber.

Your head gasket may be faulty if you notice bubbling in the reservoir or radiator, engine power loss, leakage in the tailpipe, and an overheating engine.

A Leaking Radiator

mechanic repairing a radiator

The constant changes in temperature from hot to cold may eventually wear out the radiator. 

Additionally, using low-quality coolant or adding water instead of coolant can corrode the walls of your radiator and make it rust. In the end, the radiator starts leaking.

A Faulty Cooling System

The engine is not the only component responsible for engine temperature. 

Other components that play a role are the radiator fan and a water pump that circulates the coolant. 

When these components malfunction, they affect coolant flow and cause coolant loss or engine damage. For instance, if the coolant gets stuck in the thermostat housing, it may leak to the ground.

Further, if there’s an electrical fault in the radiator fan, the temperature may increase and burst the reservoir cap. A water pump with a loose or worn-out drive belt is faulty, and it may also cause drastic coolant loss.

A Faulty Radiator Hose

man checking a radiator hose

A damaged or worn-out radiator hose may spill coolant. Such damage arises from high temperatures in the engine. When it happens, you’ll get a low coolant warning light, and failure to repair it damages the head gasket.

A Faulty Expansion Tank

A damaged expansion tank, or overflow tank, may leak coolant, causing a deficit in the radiator. The tank’s plastic housing may deteriorate and cause this leakage.

How Much Loss Is Normal?

Your car needs coolant in summer or winter. When it’s cold, the coolant prevents the liquid in the radiator from freezing, and in summer, it keeps the temperature below the boiling point.

We’ve learned that leaks may happen due to radiator or engine issues. But, some leaks may be so small they go unnoticed.

For example, even though coolant moves in a closed system, there’s an overflow reservoir. When you turn the engine off, the coolant also cools down. It contracts, and the pressure drop creates a vacuum. 

That’s why there’s an overflow tank from where more coolant flows into the radiator to fill the vacuum. Therefore, we can’t say that your coolant level can never drop. If your car doesn’t have an overflow tank, as with older cars, the system sucks up air to fill the vacuum and causes rust.

Additionally, water in the coolant turns into steam using high temperatures that cause negligible evaporation. Thirdly, as the engine gets old, coolant loss may increase.

These three issues may cause negligible evaporation that shouldn’t worry you. This drop we’re talking about should be about 0.25% every five months. If it’s higher than that, then you’re dealing with something more serious.

How to Check for Coolant Loss

As engine coolant circulates in the system, it absorbs heat from your engine and passes it to the radiator for cooling through airflow. That’s why it’s mostly water so that it can conduct heat.

Losing coolant can cause engine failure due to overheating. If you drive when the coolant reservoir is empty, you risk increasing pressure and cracking your engine.

For that reason, here are ways to check your coolant level.

  • Wait until the engine cools down before opening the radiator cap. The hot steam that gushes out of it can burn your face. Therefore, give it at least half an hour to cool down.
  • Check the level in the reservoir instead of the radiator because that’s the storage. Find out how much coolant should be in the reservoir
  • Inspect the coolant for contaminants or color change. If it’s colorless, has sludge, or rusty color, you need a mechanic. Also, pull the dipstick out and wipe it on a rag to check for coolant leakage in engine oil. If the substance is milky white, there might be coolant in the engine oil.

Since the cooling system may also cause coolant evaporation, it’s wise to check other components even as you worry about coolant level. 

Components that may cause a leak include the water pump, radiator hoses, cylinder head gasket, thermostat housing, and the transmission oil cooler. You’ll be looking for any discoloration, dampness, rust, or darkening. You can also check the ground under your car for any leaks.

Check the radiator hose for any damage that may cause leaks. If there are cracks, swellings, chafing, or rusty spots, they may be faulty. On top of that, if there are sticky spots on the cooling system hose, there may be oil contamination.

Before doing an engine pressure test, you can inspect the radiator cap for rust deposits or a brittle O-ring. Worn out sealing surface on the radiator’s neck can also make it faulty. After that, conduct a pressure test for any leaks in the engine. It should hold the pressure subjected from the pressure tester for a minute. If it doesn’t, consider replacing it.

Another test is the combustion leak test, one of the best ways to reveal a faulty head gasket or a cracked cylinder head.

After checking the coolant level and cooling system, what can you do? 

Maintenance Tips When Your Car Loses Coolant

As we stated earlier, some coolant losses are negligible. However, if your car needs a refill every so often, the problem needs fixing. Failure to catch abnormal changes in coolant levels can be costly because you’ll deal with engine failure. 

Further, when the coolant level is too low, it may damage the water pump. These issues call for cooling system maintenance so you can catch any faults that may affect coolant levels.

Let’s look at a few areas to remember.

Check the Reservoir Cap

Replace the reservoir cap after a few years or check it at least twice a year for dubber damage or rust.

Coolant to Water Mixture

Manufacturers pre-mix them, but if the seasonal changes demand that you change the water to coolant ratio to enhance performance, do it right.

Replace Damaged Radiator Hoses

Worn-out hoses will not transport coolant efficiently, and you’ll see leakage under your car. It’s one of the causes of external leaks.


Can coolant get low without a leak?

Yes, as we mentioned above, your car may have faulty components, such as the head gasket.

How often should you top up coolant?

Check the level at least once a month and top it up if necessary. But, if you notice a drastic loss, do the checks every week. Change your coolant in two years or after every 30,000 miles. The coolant level should be below the rubber. If you overfill, it may leak.

What are the symptoms of low coolant?

One inescapable warning sign is a faulty heater. The coolant powers it, and when the coolant level drops, the heater may stop working. Additionally, if there’s an abnormal drop or increase in the car temperature as you drive, your cooling system might be faulty.

The coolant warning light is the most obvious sign. It alerts you to check the coolant level when the engine overheats. You have to pull over and check it immediately or risk getting worse engine problems. But, as we said earlier, give the car time to cool down before you pop it open. 

Fuel economy is another sign that your car has a low coolant level that’s causing the engine to overheat.

Final Thoughts

Coolant level affects engine temperature. This level may drop naturally due to the high temperatures in the engine, plus the presence of an overflow reservoir.

However, the standard evaporation may increase when there are other causes. For example, a faulty head gasket leaks coolant into the engine chamber, where it burns off. Other reasons for sudden coolant loss are coolant flow disruption, faulty hoses, and radiator leakage. Which one is ailing your engine?