Every modern vehicle has three core systems that work together to keep engine operating temperatures at safe levels.

The most basic system is simple air cooling of your engine bay.

All vehicles have air intakes that work to provide airflow for the Internal Combustion Engine, but they also have grills upfront and are designed to allow air into the engine bay to exchange out hot air (as much as possible, anyway).

This is a passive system for sure, but it is still a big piece of the puzzle.

Another of these three systems is the motor oil that you put into your engine and replace on a regular basis.

A quality lubricant that moves through your engine to make sure that metal parts aren’t causing a lot of excessive friction, motor oil also works as a heat exchange medium to keep things nice and cool.

That’s definitely the secondary duty of motor oil (lubrication is the main priority), but it still helps to keep temps under control.

The biggest system to keep your engine nice and cool is the actual coolant system in your vehicle.

Pushing and antifreeze mixture throughout your car to exchange heat via the radiator so that core components don’t overheat and become damaged, making sure that your coolant levels are always consistent – and that the system is performing as intended – is a major part of responsible ownership.

What happens when your coolant looks like it has started to boil, though?

My Coolant Looks Like It’s Boiling – What’s Up?

If you’ve noticed the coolant in your vehicle begin to boil it means that there is something wrong with the closed radiator system.

Air Pockets in the Tank

The most common reason that you might be seeing air bubbles brewing up in your reservoir tank is because of air pockets that have been introduced into the system, usually during maintenance.

These bubbles are usually pretty gentle (think of it as a slight simmer versus a hard-boiled) and will – for the most part – work themselves out. Sometimes these bubbles need to push a little bit of coolant out of the expansion hose, but that’s okay as well.

Thermostat Issues

Thermostat issues are little more serious and can cause a harder boil than the gentle simmer of air pockets.

Your thermostat is responsible for opening and closing your coolant flow, allowing coolant either move through the engine system as intended or be redirected to the radiator to shed the excess heat that it has accumulated.

When your thermostat acts up it’s not at all uncommon to see the coolant in your expansion tank start to bubble up and boil. You might even start to notice a knocking or pounding sound coming from inside of the radiator because of the way that the coolant is being shuffled around.

If this is the root cause of your bubbling you’ll need to replace that thermostat ASAP. It’s not working the way it should be.

Expansion Hose Issues

Expansion hoses (used to handle overflow from the expansion tank) sometimes have tiny little cracks in them that you don’t notice with the naked eye, especially since these hoses are often made of black material – and usually pretty dirty.

If there is a split or crack in the material anywhere (even just a little bit) it’s going to introduce air into the otherwise closed system.

That air is going to get trapped in the expansion tank itself and cause the coolant within to look like it is boiling. It’s really just a lot of those air pockets we mentioned earlier being created, popped, and then created all over again because of the consistent leak.

Bad Water Pump or Head Gasket

Water pump seals that have failed or head gaskets that are on the way out can also cause issues with the sealed coolant system, introduce air into the mix, and cause the bubbling that looks like boiling.

Failed water pumps and head gaskets will allow air to enter into the intake side, getting sucked right into the coolant system, and then the air that was trapped gets released within the expansion tank.

Both of these parts need to be replaced when they are the culprit behind the boiling appearance of your coolant.

Coolant Reservoir Boiling But Engine Doesn’t Overheat

If you’ve noticed, though, that your coolant reservoir is boiling away but the temperature in your vehicle (judging by the temperature gauge) isn’t getting crazy hot – or even moving from where it normally goes when the car is running – you’re probably dealing with different issues.

Most of the Time You Need a New Radiator Cap

When you have coolant boiling and no temperature swings you usually need to swap out the old radiator caps for a new one.

Radiator caps aren’t just for keeping fluid inside of their tank while rolling down the road. These caps are designed to help pressurize the system when the vehicle is on, and if they are split – or even if the O-rings inside of them are worn out and dry – air is going to get sucked right into the tank.

If you’ve noticed that your coolant boils and then when you shut the vehicle off it looks like you have less in your reservoir than you did when you started – but nothing has spilled out on the ground or is leaking – it’s definitely your radiator.

What’s happening is the water is boiling off of the coolant mixture before the antifreeze component is, leaving you would less fluid in the system (all without showing any obvious signs of leaking).

The water in your coolant is able to boil out through the crack or leak in your cap. Replace it for a couple of bucks and you won’t have this problem anymore.

Sometimes, Though, You’re Dealing with a Head Gasket Issue

The absolute worst thing that can cause boiling in your cooling system is a head gasket leak or failure.

Head gaskets are incredibly expensive to fix, so much so that it’s not at all uncommon for the work necessary to swap out head gaskets on a used vehicle to actually total the car completely – even if everything else was in perfect working order!

The project is very labor-intensive even if the parts are all that expensive. You literally have two pull your engine apart (pretty much from top to bottom) to resolve this problem.

There are some head gasket leak fixing fluids you can try to add to your coolant reservoir to resolve this problem. Sometimes they work and sometimes they don’t.

If this is the reason for the bubbling coolant in your vehicle, though, it might be best to start shopping for another car.