What Should You Do If Your Car’s Coolant is Boiling?

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coolant boiling

What happens if your coolant begins to boil? Well, let me tell you what won’t happen: Your car won’t explode into flames and take out half the block with it. The radiator will not suddenly increase 50x in size, and the cooling fan will not pull your engine out through your hood.

Your car will just overheat – plain and simple. This can cause serious damage to pistons, cylinder walls, valves, and bearings, as well as significantly reduced gas mileage (due to cooling system drag), increased oil temperatures (which can cause problems with lubrication), and the possibility of your car simply shutting down altogether.

Why is My Car Coolant Boiling?

Car coolant boils when the engine temperature rises above its boiling point. So all causes for engine overheating are also the reasons why coolant would boil. To read a detailed article on the causes of engine overheating, you can click here.

Failure of cooling systems will lead to a hot engine. However, if the anti-freeze of your is boiling, it might be due to the following reasons:

  1. Defective Radiator Cap
  2. Blown Head Gasket
  3. Faulty Water Pump
  4. Bad Thermostat
  5. Clogged Radiator
  6. Air Pockets in the Radiator
  7. Low Coolant Level
  8. Turbocharged Cars

Let us see the reasons for coolant boiling in detail:

1. Faulty Radiator Cap

A faulty radiator cap will cause the coolant of your car to boil. One way this could happen is if the pressure cap blows off under high boost, allowing unmetered air into the engine.

This causes it to overheat and produce steam and copious amounts of smoke. The throttle can be floored, and the engine will keep going, but it’ll have zero power as water doesn’t compress!

If your temperature gauge is moving, then make sure to check out the radiator cap.

2. Blown Head Gasket

A blown head gasket is a leak in the cooling system between the cylinder heads. This can allow combustion gases, engine coolant, and oil to mix. This mixing causes the car to overheat due to the loss of coolant.

3. Faulty Water Pump

Water pump failure also causes the coolant to overheat and therefore build up pressure. If you’re familiar with this type of incident, then you’ll know that if the water pump fails, it usually continues to lose coolant even after the engine is shut off.

This is because the water pump has failed catastrophically and can no longer create enough pressure to push coolant through the system even when the engine is not running.

4. Bad Thermostat

A bad thermostat valve is responsible for opening or closing the flow of hot coolant between the engine and the radiator. The purpose of the thermostat valve is to maintain a water temperature near 55°C, which ensures efficient engine combustion.

A faulty thermostat can get stuck in one position allowing too much coolant to circulate into the engine, causing it to overheat. This may lead to the boiling of engine coolant as well.

5. Clogged Radiator

Overheating, boiling over coolant, and steam coming from the hood are signs of a clogged radiator. Clogged radiators can be caused by debris blocking water passages inside the radiator or an overheated system that has not been properly bled.

Debris in a radiator may come from a variety of sources. Newer vehicles use plastic radiator tanks. Plastic produces microscopic fibers that can break off and become trapped in the radiator’s cooling passages, especially near seams on the inside of the tank.

6. Air Pockets in the Radiator

Trapped air in the radiator will cause it to boil. That’s how simple it is. If there are bubbles in the cooling system, they need to be removed or have problems. The car will get hotter and run less efficiently, not to mention the risk of overheating.

But sometimes, the air pockets aren’t that easy to get rid of. Here’s what you can do about it.

First, identify why bubbles are in our water system in the first place: Air has most likely seeped into the cooling system due to a faulty radiator cap or damage caused by overheating (thermal expansion).

Replacing the cap immediately solves this problem. The same goes for a leaking hose, fault in the radiator, or other components in your cooling system.

7. Low Coolant Level

If your coolant is boiling, it might be due to its low level. Your only option in this scenario is to pour in more coolant to top up your level.

You may have a little warning that the level is low by a low coolant light on your dashboard, but if you don’t, then what’s going to happen is that your engine will start boiling and spewing out steam – which will show itself as a big plume of steam from your engine.

Leak repair fluid is available in the market to help you maintain the necessary fluid level in the coolant reservoir.

8. Turbocharged Engine

It can also happen when the engine is racing (e.g., at least one turbocharged car). This is because the input boost pressure of a compressor increases with temperature, and if it reaches its critical temperature, it will fail catastrophically – in an instant!

This usually takes out the rest of the powertrain and produces a cloud of smoke from the engine compartment. In this case, a full rebuild is also necessary.

It doesn’t fully fail in some cases and allows the turbo or supercharger to continue working at reduced efficiency until its next thermal cycle. It tends to happen under high load on a hot day when a fresh start of coolant hasn’t been pumped through the system.

It also happens with a faulty thermostat that doesn’t open up when needed, allowing the coolant to overheat and boil.

How To Fix Coolant Boiling?

The best way to avoid this is to ensure that the coolant level in the expansion tank is maintained and that you use a quality antifreeze with corrosion inhibitors.

These two things will protect your cooling system no matter what happens. Drive carefully when the engine has started to overheat so you can reduce speed and get it to a safe place.

And, of course, a comprehensive engine protection package using a water-methanol injection system with effective cooling management will reduce the chances of this occurring!

Some aftermarket heat exchangers allow you to fit an externally mounted radiator to your standard radiator. These are more efficient at cooling and can’t trap coolant from leaking from the standard radiator.

You can then fit a larger capacity pump and cooling fan to ensure that the coolant is pushed around the whole system under high demand.

You can also fit an oil cooler to ensure that the engine oil is cooled if you’re running enhanced turbocharger or supercharger boost systems.

These are highly effective modifications and allow any vehicle of almost any age to run at maximum power. It is really worth fitting one of these if you like your car, as it will significantly prolong its life.

How Air Enters the Cooling System?

Air enters the cooling system through below means:

  1. Leaked Coolant Reservoir Hose
  2. Loose Pressure Cap
  3. Leaky Head Gasket

1. Leaked Coolant Reservoir Hose

The most common cause of bubbles in the radiator is a crack in one of the coolant hoses. If these aren’t perfectly clear, they can trap air inside, which will work its way into your cooling system and form bubbles under pressure. The result? You guessed it, a bubble in your radiator.

A crack can hide inside a hose for a while before it starts to leak and make bubbles. It’s only when the pressure gets high that they start to appear. That means if you just had this checked out, there is no guarantee that there aren’t any air pockets hiding in the cooling system.

2. Loose Pressure Cap

The problem is that when water boils, it becomes steam – this makes bubbles that rise to the top of your container, pushing out the cooler fluid underneath. So when your coolant boils, it’s pushed out from under your radiator cap. This is a bad thing because if you’re not careful, your car engine will overheat and be badly damaged – in the extreme case, this can even cause an explosion.

3. Leaky Head Gasket

A leaking head gasket can also allow air bubbles to enter the cooling system. The cooling system pressure is not maintained. And to circulate coolant, a specific level of pressure must be available. This leads to overheating of the engine and boiling coolant in turn.


Car coolant boiling is a common problem in vehicles. It is caused when the engine overheats due to being stuck in traffic. What happens is that the coolant boils in an enclosed space with no way of venting out, causing pressure to build up and disrupting the flow of fluid through the engine block.

The expansion forces some of it past the cylinder head seals into the cylinders, where it mixes with the engine oil and causes a cloud of steam to be ejected from the exhaust.

There’s only minor engine damage in many cases, but on some occasions, a warped cylinder head or a cracked block can result. This is a major mishap that necessitates a full stripdown and rebuilds of the affected area.

The most severe case is when it impacts a cylinder head or piston directly in a racing engine where the result can be fatal. In all cases but directly impacting a piston, a replacement cylinder head and block is required as the process of machining one to take a replacement piston can be quite expensive.

Frequently Asked Questions

Is it bad if the coolant of my car is boiling?

Yes, it is really bad if your car’s coolant is boiling. Your car’s coolant (also known as antifreeze or anti-boil) works to remove heat from your engine. This liquid is circulated through the engine block and then into the radiator.

When you turn on your air conditioner, this liquid also goes through a different path to help the engine cool down. Your car must operate within a specific temperature range at all times, or serious damage can occur to the engine.

What is the boiling point of the coolant?

The boiling point of ethylene-glycol based fluid is around 248°C (480°F) and the boiling point of a propylene glycol one is 205°C (401°F). These are both extremely high temperatures, and you wouldn’t want to be standing in front of an open, boiling radiator. If the temperature drops just a little, then your anti-freeze will start to freeze, and that’s not so good either.

A mixture of water and ethylene glycol has its freezing point at around -37°C (0°F), but a mixture of water and propylene glycol is -62°C (-80°F). This means that if you have a car with a 100% propylene glycol anti-freeze, then it’s almost impossible for the fluid to freeze unless the outside temperature drops below -50°C (-58°F), constraining your engine.

How can excessive heat affect the working of a car’s engine?

Excessive heat can affect the working of your car’s cooling system in a number of ways. Engines generate a lot of heat as part of their normal operation, and if this is allowed to build up in the engine compartment it can have serious consequences.

Engines run at optimum efficiency when they are between 90°C and 110°C – any hotter and they operate less efficiently making your car work harder and use more fuel. If the temperature in the engine compartment reaches 130°C, then there is a serious risk of fire.

As well as being dangerous, an overheating car can also cause permanent damage to the engine if left in this condition for long enough.

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