Why Is My Coolant Reservoir Boiling: How To Fix?

Coolant fluid is an important element of the car & it’s reserved in the coolant reservoir. The main job of coolant is to deal with the heat by absorbing & reducing from the engine.

If you’re noticing your coolant is boiling & wondering – why is my coolant reservoir boiling, this article is for you. In this article, I’ll be explaining why it’s happening & how you can fix it. 

Car Coolant: Why Do You Need Coolant In Your Car?

A car engine produces lots of heat burning fuel while operating. This heat needs to be dismissed via the cooling system of the car. The radiator is one of the central cooling components of a vehicle & liquid coolant is the main ingredient of it. 

Liquid coolant is reserved in the coolant reservoir. This coolant is managed by the radiator & other radiator components. Here is how a car engine heat is dismissed (with the help of coolant liquid):

  • The coolant liquid travels through the hoses around the engine from the radiator & absorbs the engine’s excess heat. 
  • The coolant returns to the radiator & thin metal fins help in releasing the heat from the coolant. 
  • This time, the radiator fan/s helps with airflows through the grille & makes the environment cooler. 

In summary, the coolant plays a great role in reducing engine excess heat. This work of coolant also increases the lifespan of other heat-vulnerable components like the head gasket. 

Why Is My Coolant Reservoir Boiling?

As you already know, the coolant & coolant reservoir job is to keep the engine cool via absorbing heat from it. However, sometimes the coolant becomes too hot than usual & starts to boil. 

Still, let me clarify the other coolant reservoir boiling factors below: 

  • Faulty Head Gasket
  • Faulty Radiator Cap
  • Faulty Radiator Fan
  • Faulty Thermostat 
  • Old/Faulty Radiator 
  • Old Coolant Fluid

With short introductions of the factors mentioned above, you’ll understand the situation better. Here we go:

Faulty Head Gasket 

There can be many gaskets in a car & the most crucial one is the ‘Head Gasket’. This head gasket is designed in a way to seal the firing pressure of the cylinder. 

Besides, it prevents oil & coolant from leaking into the cylinders. You’ll face many problems along with the heated coolant reservoir if your head gasket is faulty. Here are the symptoms of a faulty head gasket: 

  • White smoke from the tailpipe
  • Bubbling coolant reservoir (due to boiling coolant)
  • Unexpected low level of coolant (due to coolant loss)
  • Overheated engine

If you notice these symptoms mentioned above, suspect the head gasket for the coolant boiling issue.  

Faulty Radiator Cap

Though a radiator cap costs around $30 to $50, it does a great job sealing the cooling system & ensuring it remains pressurized.

A pressurized cooling system is important for the radiator to operate properly & prevent the coolant from boiling.

A faulty radiator cap is enough to create a situation of boiling coolant. Other than that, here are the symptoms of a faulty radiator cap:

  • Irregular pressure of the cooling system 
  • Leaking coolant fluid
  • Boiling coolant fluid 

If you notice these three symptoms mentioned above, suspect the radiator cap for the coolant boiling issue.  

Faulty Radiator Fan

One of the most important radiator’s components is the radiator fan. Depending on your car model, you may have a single or double radiator fan setup. 

Whatever your fan setup is, the main objective of this fan is to cool down the coolant liquid. If somehow it gets defective, you may find your coolant is boiling. 

Faulty Thermostat

The thermostat plays an important role in monitoring the coolant temperature & regulating fluid. It gets wide open for the fluid to come & reduce engine temperature. 

Sometimes, a faulty thermostat malfunctions & doesn’t open well. In that case, you may notice your coolant reservoir is boiling along with some weird noises. 

Old/Faulty Radiator 

The radiator is one of the most important components of the cooling system. It manages the coolant fluid & reduces engine temperature. 

A very old or faulty radiator won’t operate fluently, resulting in a boiling coolant/coolant reservoir. Here are the symptoms of a bad radiator: 

  • Unexpected rise & fall in the instrument cluster temp gauge.
  • Finding coolant fluid in the ground or under the vehicle.
  • Dust & rust in other cooling components
  • Boiling coolant

If you find four symptoms mentioned above, suspect the radiator itself for the coolant boiling issue.  

Old Coolant Fluid

You already know why coolant fluid is used for – yes, for absorbing heat from the engine. As the coolant ages, it loses its capacity of holding/absorbing heat & carrying it to the radiator. 

If you’re using coolant even after 60,000 miles (before the first coolant replacement) or 30,000 miles(after the first coolant replacement) of usage, this might be the reason you have a boiling coolant issue. 

General Advice: What Should You Do If Coolant Is Boiling?

Boiling coolant can represent lots of potential issues with your car. That’s you shouldn’t drive your car (other than driving it to the auto shop) until the actual problem is revealed & solved too. 

How Serious Is The Coolant Reservoir Issue For My Car?

Coolant’s job is to absorb & release heat collected from the engine. If the coolant isn’t doing its job properly & boiling instead, it will badly impact your engine components. 

For example, the head gasket is one of the most expensive parts of the engine & it costs hundreds of labour to replace it. Boiling or bubbling coolant can be responsible for a blown head gasket.

Like the head gasket, other engine components can become faulty if the boiling coolant remains unchecked & unfixed. Follow the next segment to know how to fix boiling coolant reservoir. 

How Can I Fix My Boiling Coolant Reservoir?

As you already know why your coolant reservoir might be boiling, start your inspection via checking those engine components. 

Before starting the operation, make sure you have the equipment mentioned below: 

  • Technical Service Bulletin (TSB)
  • Screwdriver Kit According To Your Vehicle 
  • ¼-inch Ratchet, Extension & Socket 
  • Safety Goggles & Gloves 

Considering you’re ready with the proper equipment, let’s get started with the troubleshooting methods of fixing the boiling coolant issue. 

Replace/Flush Coolant Fluid

It’s recommended you should change your coolant every 30,000miles for optimum performance. However, if your vehicle is brand new & the coolant was never replaced – consider flushing/replacing it after the first 60,000miles. 

Note: You can check your truck’s TSB (Technical Service Bulletin) for the exact mileage count before you should replace/flush coolant fluid. 

Replace The Radiator Cap

The radiator cap is a cheap component of the radiator & it’s very easy to replace it. If you’re facing a pressure issue & the engine is acting up, your radiator cap might be faulty. Consider replacing the radiator cap. 

Caution: You should never touch or open a hot radiator/radiator cap as it might cause severe skin damage. Wait until the engine is fully cool & then you can replace the cap.

Replace The Thermostat 

Sometimes, a faulty thermostat can be responsible for the boiling coolant fluid. Here’s how you can test whether your thermostat is okay or faulty: 

  • Step 1: Check your TSB & see what are the opening temperatures of your thermostat. 
  • Step 2: Open the hood, locate the thermostat & remove it gently.
  • Step 3: Pour some water into a steel bowl & put the thermostat in it. Make sure the thermostat is properly covered with water. 
  • Step 4: Use a thermometer in the corner of the steel bowl & it should not touch the bowl corner. 
  • Step 5: Release the heat & wait until water reaches the opening temperatures (mentioned in the TSB). 
  • Step 6: if you find your thermostat isn’t opening or not opening just like it should be, consider replacing it. Otherwise, your thermostat is fine. 

Considering your thermostat is faulty, here’s a step by step guide on how to replace the thermostat:

  • Step 1: Drain the coolant from the radiator
  • Step 2: Open the hood, locate the radiator cap & remove it
  • Step 3: Locate the thermostat & remove it. You can use a ¼-inch ratchet extension & socket to aid the removal process. 
  • Step 4: Replace the thermostat with a new one
  • Step 5: Do the rest in a reverse manner & you’re good to go.

This is how you can replace your thermostat & see if your boiling coolant issue is solved. 

Replace The Radiator Fan

Replacing the radiator fan is the last thing I would recommend doing on your own. Here is the step by step guide on how to replace the radiator fan: 

  • Step 1: Remove the air filter after opening the hood
  • Step 2: Unplug any electrical connections with the radiator 
  • Step 3: Unscrew the radiator 
  • Step 4: Remove the battery 
  • Step 5: Remove the cover of the radiator hose & remove the radiator hose afterwards. 
  • Step 6: Finally, remove the radiator fan & replace it with a new one. 
  • Step 7: Do the rest in a reverse manner & you’re good to go. 

This is how you can replace your radiator fan & see if your boiling coolant issue is properly solved. 

Replace The Head Gasket

Replacing the expensive head gasket by an individual can be a very tricky job & I wouldn’t recommend anyone (with low or entry-level knowledge) replacing their head gasket on their own.

Replace The Radiator 

Like the head gasket, I wouldn’t recommend any general user to replace their radiator on their own.

How Much Does It Cost To Fix Boiling Coolant Reservoir?

Depending on your replacement parts, the cost of fixing a boiling coolant reservoir may vary from truck to truck. For your clearer understanding, here is the price chart of potential replacement parts:

  • Radiator Cap Costs $10 to $50
  • Single Radiator Fan Costs $30 to $50, and Double Radiator Fan Costs $100 to $150
  • Thermostat Costs $30 to $50
  • Coolant Fluid Costs $100 to $150
  • Head Gasket Costs $700 to $850
  • Radiator Coss $250 to $450

Don’t forget to add an extra $75 to $150 per hour labour cost if you’re getting these works done from an auto shop. 

How To Prevent Coolant Reservoir From Boiling? (In Future)

As you already know, there’s no ‘exact’ reason or component that causes boiling coolant reservoir. Still, here’s how you can prevent the head gasket from blowing & radiator from being faulty: 

Blowing Head gasket prevention: 

  • Keep an eye on the coolant level & ensure it’s at the proper level.
  • Check your ground for coolant leakage. If you find coolant on the ground, have your vehicle inspected by professionals
  • Check your temperature gauge & consult a professional technician to inspect your vehicle. 

Faulty Radiator Prevention: 

  • Replace the radiator hoses every 36,000 miles. 
  • Have your coolant flushed every 30,000 miles.


In summary, the coolant of your car works as a heat absorbing element & reduces the overall heat of the engine. If your coolant fluid is boiling, chances are they’re not working well due to various factors such as – faulty radiator cap, fan, thermostat, head gasket & radiator. 

I hope this article was helpful enough to satisfy your query – why is my coolant reservoir boiling. As I’ve mentioned the fixes as well, you can now decide whether you should fix them on your own or hire a technician.

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