Coolant Boiling in Reservoir: The Causes and How to Fix It

(Last Updated On: March 14, 2024)

Should your coolant be boiling in the reservoir, the cause of this could likely be a faulty cooling fan, a damaged head gasket, or several other issues. You may detect a heated odor wafting from the vents or observe steam emanating from beneath the bonnet when the coolant boils.

This article outlines the various causes of coolant boiling in the reservoir and possible fixes. We’ll explain how to go about the simple fixes and deal with the complex ones. In addition, we’ll tell you how to prevent engine coolant from boiling. 

Otherwise, read on to know what to do when your engine’s coolant boils over. 

Here is where that explain the causes of coolant boiling in reservoir
In the article, we’ll give you information about coolant boiling in the reservoir.

What should i do if the coolant boils?

Engine coolant keeps the engine working at the correct temperature 
Engine coolant/antifreeze works in a sealed network of hoses that pump it around the engine.

Engine coolant/antifreeze works in a sealed network of hoses that pump it around the engine. The coolant goes through the various components, including the cylinder heads, gaskets, and engine block, to keep the engine working at the correct temperature. 

However, when air enters this sealed system, it messes with the pressure causing bubbling and blockages. 

When you notice that your car’s coolant is boiling, you should pull over immediately. However, if it’s not safe to stop, you can turn on the heater, turn off the air conditioner, and roll down the windows. 

Then, continue driving steadily while reducing stop and go as much as possible. When you can come to a stop, open the bonnet and wait for the engine bay to cool down before checking out the problem. If you are unsure of the problem, have the vehicle towed to a mechanic. 

Nevertheless, if you can see coolant boiling out of the reservoir cap and hot water under the hood, you are experiencing a few issues with coolant components. 

Quick fix 

  • Open the radiator cap slowly, making sure to keep your face away from possible sputters of hot water or steam. 
  • Check the coolant level and refill it if it’s below the standard level. If you don’t have coolant, plain water will do just fine for the time being. 
  • Allow the engine to cool further, then replace the radiator cap. 
  • Once you get the vehicle back on the road, be sure to check in with a technician to identify any causes of coolant boiling to prevent reoccurrence. 

The signs of my car coolant boiling 

These are some of the signs that your engine coolant is boiling over.

  • Reduced engine power
  • Steam coming from under the bonnet
  • A hot smell from the vents 
  • A high-temperature reading on the temperature gauge and an accompanying warning light on the dash 
  • A sound similar to boiling water on a stove coming from the bonnet. 

The causes and how to fix it 

causes lead to the coolant boiling in reservoir
Six causes lead to the coolant boiling in reservoir.

A failing cooling fan 

The fan keeps the radiator cool when the vehicle is not moving. It is usually hooked to the radiator through a network of wires. These wires are prone to corrosion over time. When the wires wear out, the radiator will overheat because the fan does not provide sufficient air circulation when the vehicle stops.

Some of the reasons for a failing cooling fan include a blown fuse, broken wire, or bad relay. 

How to diagnose and fix 

In newer cars, you need to ensure that the radiator cooling fan has stopped working.  To this end, you’ll want to turn on your engine and allow the vehicle to idle. Then, please turn on the air conditioning to its maximum setting. This step will start the cooling fan immediately. 

In an older vehicle, you may have to wait for 15-20 minutes for the engine to warm up and reach the ideal temperature. 

If the fan comes on, but it’s not running correctly, and the engine is hot, it could be that the temperature switch is not working, or you could have blown a fuse. A bad temperature sensor will trigger the check engine light. Hence, you will have to scan the trouble codes and identify the proper fix. 

For a blown fuse, look for a fusible link to the circuit. It will be a piece of wire in an insulated piece of rubber. Now, grab the wire’s ends that go into the fusible link and stretch it. If it stretches, it’s likely broken and will need to be replaced. 

In some cases, the fan fuse might be blown, which will trigger the breaker. You will need to replace the fuse and reset the breaker. 

A video about how to repair a cooling fan.

Repair and replacement cost 

Fuse and broken wiring cost $10 to $20 for diagnosis and replacement, while the temperature sensor will cost $120 to $160 to replace, depending on the vehicle type and model.  

Blown head gasket 

A blown head gasket can lead to coolant boiling in reservoir
Another cause of coolant boiling in the reservoir is a failed head gasket.

A failed head gasket is another cause of coolant boiling in the reservoir. This problem is easy to diagnose since you’ll see foam in the expansion reservoir or air bubbles in the coolant. When a head gasket blows, it prevents proper working of the water pump, thus inhibiting its ability to circulate the coolant through the system. 

As a result, the vehicle will overheat on idle

You should not open the radiator cap when the engine is still hot. As we stated before, you can get burns from boiling water under pressure. 

Again, if you notice any of the signs mentioned above, it’s best to pull over immediately. After which, you can perform a simple DIY fix like we discussed earlier or have the vehicle towed. 

Now, a head gasket replacement can be pretty expensive. However, you can fix the same problem permanently with the help of leak repair fluid at home. A suitable sealant like K-Seal will come in handy. 

All you need to do is add the sealant overflow/expansion tank and then top up with the right amount of coolant. This should be done when the engine is idling at a normal temperature. 

A video about how to fix a blown head gasket.

Repair costs 

Replacing a blown head gasket can cost you $1000 to $2000 in labor and parts. However, if you choose to use leak repair fluid, it will cost around $12 to $20

Read more: Engine Hot AC Off: What Does It Mean And How To Fix It?

Failed thermostat

A thermostat failure is pretty easy to diagnose and fix at home
The thermostat works to regulate the flow of coolant around the engine.

The thermostat is an essential part of how your vehicle operates. Its work is to regulate the flow of coolant around the engine. A thermostat failure is pretty easy to diagnose and fix at home. 

However, you still need to know the symptoms to look out for. 

  • High-temperature reading on the gauge
  • Overheating engine 
  • Erratic temperature changes 
  • Coolant leaks under the vehicle or around the thermostat 

Instead of trying to repair the thermostat, it’s always best to replace it since it’s affordable. 

After you suspect that the thermostat has failed, it’s still important to make sure that indeed the thermostat is causing your engine to overheat and the coolant to boil. 

How to diagnose 

  • Remove the radiator cap.
  • Idle the engine. If coolant starts to flow right away, it means the thermostat is stuck open. When coolant does not start flowing, wait for the engine to warm up. The coolant should start to flow in ten to twenty minutes. The thermostat is stuck closed if the coolant does not flow, but the temperature gauge rises. 
  • However, if the coolant is flowing normally, another problem could be causing the coolant to boil. 

If you’ve verified that the thermostat is stuck open or closed, it will need a replacement. 

You will need:

  • Rags 
  • Socket/ratchet set 
  • Hose clamping pliers
  • Drip pan
  • Plastic scraper
  • New thermostat 
  • RTV sealant 
  • Coolant 
  • Gasket 

How to:

steps to replace the thermostat
Nine steps to replace the thermostat.
  • Locate the old thermostat near the top of the engine. It should be under a gooseneck housing attached to the upper radiator hose. 
  • Remove the thermostat by loosening the bolts that hold the gooseneck in place. 
  • Use the plastic scraper to remove the old gasket and old sealing compound. It’s also best to clean the gooseneck sealing surfaces and engine and dry the surfaces with a rag. 
  • Coat one side of the gasket with RTV sealant (unless you have a self-adhesive sealant that does not need sealant). 
  • Install the gasket and the new thermostat into the recessed part of the gooseneck or the engine.
  • Bleed air from around the top.
  • Hold the thermostat in place on the gasket if you’re working with a self-adhesive gasket. Then, apply a little RTV sealant. 
  • For an old gasket that used an O-ring, you have to lubricate the new one with a new coolant before inserting it. 
  • Replace the gooseneck and top off the coolant reservoir using the correct amount of coolant. 

Replacement cost 

While the cost of a new thermostat is between $20 and $40, you will pay $140 – $300 for labor at a professional auto repair shop. 

Faulty radiator cap 

A bad radiator cap causes the coolant boiling in reservoir
A bad radiator cap can cause problems to other parts of the cooling system.

The radiator cap maintains the prescribed pressure of your vehicle’s cooling system. A bad radiator cap can cause problems to other parts of the cooling system since the system cannot operate efficiently without boiling the coolant. 

Common radiator cap failure signs

  • Overflowing reservoir
  • Overheating engine
  • Leaking coolant 
  • Air in the cooling system 
  • Low coolant level 
  • Radiator hose collapses 

The radiator cap comprises three seals that may fail independently. These seals are the main seal, pressure seal, and return seal. 

To identify a faulty radiator cap, look for bubbles or coolant leaks from the hoses and the cap. You can also use a radiator cap test kit to look for pressure leaks and ensure that the system operates correctly. 

After verifying you have a faulty radiator cap, it will need replacing.

You will need:

  • Rags/protective gloves 
  • New radiator cap 
  • Drip pan 
  • Socket/ratchet set 
  • Towels
  • Fresh coolant 

How to:

How to change the faulty radiator cap
How to change the faulty radiator cap.
  • Turn your car off and allow the engine to cool down. 
  • Locate the radiator cap and open it by unwinding it. Remember to use a rag or protective gloves to remove the cap since it is most likely hot. 
  • Access the radiator by removing the mounting bolts. 
  • After accessing the radiator, loosen the radiator drain plug. Here, you want to flush out the old coolant, so place a drip pan to catch the coolant. 
  • After all the fluid drains, remove all other mounting bolts from the reservoir and follow by removing the coolant overflow tube. Ensuring that all liquid is flushed out prevents damage to the radiator by preventing coolant from turning into sludge. 
  • Re-assemble the parts and ensure you tighten the mounting bolts. 
  • Replace the radiator cap with one that matches the pressure rating of the old radiator cap. 
  • Now, refill the radiator with coolant and wind back the radiator. 
  • Start the engine and test drive to see if the problem persists. Coolant boiling in the reservoir should not be a problem anymore. 

Replacement costs 

The radiator cap is probably the cheapest part to replace. It costs $5 to $25 for the radiator cap and only $10 to $50 for labor. 

Insufficient coolant/bad mixed coolant 

Low coolant level is usually an indicator of underlying problems.
Low coolant level is usually an indicator of underlying problems.

Low coolant level is usually an indicator of underlying problems like faulty radiator cap, blown head gasket, or faulty water pump. It can also indicate that you have damaged radiator hoses. 

The low coolant warning light will come on when you’re low on coolant. The next thing you should do is read the trouble codes to identify the problem. 

If you find that the coolant is low without any leaks, all you need to do is add coolant to the “full” level when the engine is cold. Before refilling the reservoir, remember to mix it with plain water in a 50/50 ratio. 

If you have mixed coolant by accident, you’ll need to flush out the system and add fresh coolant.

Coolant refilling cost

An average coolant change costs $100 to $200. This is because it mainly involves a coolant flush. 

Read more: Can You Mix Red And Green Antifreeze?

Bad water pump 

A failing water pump will cause the engine to overheat, resulting in damage. 
The water pump takes water throughout the cooling system and around the radiator.

The water pump takes water throughout the cooling system and around the radiator. It also maintains water at the optimal temperature and cools it down when it gets hot. A failing water pump will cause the engine to overheat, resulting in damage. 

Equally important is that the water pump lasts for 60,000 to 90,000 miles. For this reason, it’s best to replace the water pump and the timing belt when it hits its assigned miles instead of waiting for it to fail. 

Signs of failing water pump

  • Coolant leaking 
  • Deposit or rust build-up 
  • Whining noise when you accelerate 
  • Overheating engine 
  • Steam from underneath the hood 

As soon as you notice these signs, you’ll need to take the car to a mechanic’s shop. This is because diagnosing a bad water pump is a long process that requires professional knowledge. 

In most cases, a bad water pump requires replacing the belt drive system alongside the water pump. That’s why a professional auto technician best does it. 

Replacement costs 

The average cost of replacing a water pump is approximately $550. The price depends on the vehicle type and model. If you’re replacing the timing belt and timing chain and gear set, the replacement cost can be as high as $900 to $1000.

How can i prevent the coolant from boiling in the future?

Cooling system maintenance is the best way to prevent coolant from boiling. It helps to change the coolant every 24 months and maintain the engine’s cooling system annually. 


Are you ready to tackle coolant boiling in reservoir? We have discussed the various reasons behind this problem and how to fix the issue. It’s down to you to identify the cause and the fix that applies to your car. Now, you can get back on the road as soon as possible. 

Read more: My Car AC Sometimes Works Sometimes It Doesn’t: Why and How to Fix

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