Why is My Coolant Reservoir Boiling?

A boiling coolant reservoir is a common problem most car owners face, and an overheated engine usually causes it. But what exactly causes the engine to overheat, and what can you do to stop or prevent it from happening too often.

Keep reading to learn how to troubleshoot and fix this problem.

Why Your Coolant Reservoir is Boiling

There are two culprits behind a boiling coolant reservoir. They are:

  • The presence of air pockets in your coolant system. Trapped air in the cooling system can cause the engine to overheat.
  • The increased pressure in the cooling system. This is caused by either trapped air or faulty parts in the cooling system.

Most cars operate through heat combustion and are driven in various climates. Cars have a coolant system to manage the temperature of the engine.

Most cooling systems include:

  • A thermostat.
  • Coolant pump (mechanical or electric).
  • Expansion tank.
  • Engine or radiator fan.
  • A temperature sensor (engine control/indicator).

Air pockets and the increase or loss of pressure are caused by faults in the components of your cooling system, and they cause your coolant reservoir to boil or produce bubbles.


Common Faults in the Cooling System That Cause Boiling Coolant Reservoir

Here are some common faults in your cooling system that could cause a boiling or bubbling coolant reservoir. You will also find out how you can identify each one.

Failing Cooling Fan

A cooling fan keeps the radiator cool when the car moves, but if the cooling fan fails due to corrosion, broken wire, or a blown fuse, the coolant reservoir will begin to boil.

Corrosion or broken wire will not allow the fan to come on, but if the fan is spinning but the temperature is still high, you may have a blown fuse.

Faulty Radiator Cap

A radiator cap maintains the pressure of the vehicle’s cooling system at the prescribed level, and it also allows the coolant to flow into the expansion tank.

Once the cap is faulty, it allows air to enter the cooling system, disrupting the cooling pressure and causing the cooling reservoir to heat up and boil.

You can tell that a faulty radiator cap is the source of the problem is the presence of bubbles in the cooling reservoir.


Bad Thermostat

The work of the thermostat is to regulate the flow of coolant in and out of the radiator. The thermostat must open and close at the right intervals to do this work properly.

When it opens, hot coolant escapes the thermostat, and when it shuts, it traps heat within the engine. A faulty thermostat that doesn’t open at the right time causes the coolant reservoir or radiator to overheat and boil.


Faulty Head Gasket

A head gasket provides the seal between the engine block and the cylinder heads. Worn-out gaskets can allow air to pass through the cylinder heads into the cooling system.

A sign that faulty head gaskets cause a boiling coolant reservoir is when the bubbling begins as soon as you start your car.


Faulty Water Pump

The water pump helps circulate antifreeze or coolant through the radiator’s cooling tubes, where it cools and flows to the engine components.

If the water pump gets faulty, the air is trapped in the pump intake, and this causes the coolant reservoir to boil.


Leaky Coolant Reservoir Hose

The coolant reservoir hose connects the radiator to the coolant reservoir. If this hose is leaky or worn out, air can slip into the hose and cause the reservoir to boil.


Simple Ways to Fix a Boiling Coolant Reservoir

You can take a few simple steps if your coolant reservoir is boiling. Here:


Flush and Refill the Coolant Reservoir

If you notice air bubbles in your coolant reservoir, you can thoroughly flush and refill the reservoir.

Make sure you allow air pockets to escape the cooling system by running the engine without the radiator cap for fifteen minutes after refilling.


Promptly Replace any Damaged Parts

You can trace most of the causes of cooling reservoir boiling to faulty cooling parts. If you are a handyman, purchase the damaged parts at any hardware store and fix them promptly.

Some stores provide seals to prevent air from entering through leaky hoses and faulty radiator caps.

Note: If you do not have the skills or the tools to fix damaged parts, employ the services of an affordable garage rather than run the risk of damaging your vehicle.


Top-up Coolant Liquid in the Right Ratio

Occasionally, a boiling cooling reservoir results from an inadequate coolant level. When this happens, your car will give a low coolant warning light.

Ensure that the low coolant level is not caused by mechanical problems such as a leaky coolant hose or faulty Radiator cap. If it isn’t, top up your coolant.

Don’t forget to mix the coolant with plain water in a ratio of 50:50. If you accidentally pour the wrong coolant concentration into your car engine, flush out all coolant immediately.


Simple Steps to Prevent your Coolant Reservoir From Boiling

A simple maintenance routine can prevent most faults that cause a coolant reservoir to boil. Some of the steps you can take include:


  1. Check your coolant level regularly and top it up if necessary.
  2. Always check for leaks in the radiator caps, coolant hoses, head gaskets, etc.
  3. Watch out for discoloration in your coolant as they are a sign of corrosion. Corrosion can lead to faults in your coolant system.
  4. Have your coolant system flushed and refilled every five years or after driving 30,000 miles.
  5. Arrange for regular cooling system testing, especially during freezing winter months.



A boiling coolant reservoir is inconvenient, but you can do something about it. Use this guide to identify the cause of the problem, get the right materials to fix it, and observe a good maintenance routine afterward.

Author Bio

With over a decade of writing helpful articles on DIY fixes, Jude writes for numerous blogs including roadabletimes.com. Jude has a unique voice that shines through in her writing, and always breaks down complex projects so everyone can DIY!