Coolant Reservoir Boiling But Engine Doesn’t Overheat (Why & How To Fix)

The coolant absorbs the heat from the car engine to prevent the engine from overheating. So, the coolant has prevented the engine from overheating, but as it absorbs too much heat, it may become too hot to start boiling.

Factors causing the coolant reservoir to boiling are faulty head gasket, radiator cap, radiator fan, thermostat, old and faulty radiator, or old coolant fluid. 

Head Gasket

The head gasket is a seal to the engine block and the cylinder head. A faulty head gasket may cause the coolant reservoir to boiling, white smoke from the tailpipe to appear, or an overheated engine.

Radiator Cap

The radiator cap acts as a valve by adjusting the coolant pressure within the radiator. As it becomes faulty or malfunctions, it may increase the coolant pressure and cause the coolant to boil and even leak if the pressure gets even higher.

Radiator Fan

When the hot coolant goes to the radiator, the fan releases the excess heat into the atmosphere. As it becomes faulty, it won’t be able to release enough excess heat and may cause the coolant to boil.


The thermostat monitors coolant temperature and regulates the coolant that flows into the engine. Thus, when the engine is cold, its bypass valve remains open. 

But as the coolant absorbs the heat as it gets into the engine, this will cause the bypass valve to close, and the main valve will open. A faulty thermostat may cause an imbalance as the valve may not open properly, thus creating an imbalance with the mechanism of the coolant to absorb heat from the engine.

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The radiator is where the hot coolant will flow and release excess heat from the coolant through the radiator fan. It may cause the coolant to boil if it doesn’t participate in releasing the excess heat from the coolant absorbed from the engine.

Old Coolant Fluid

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As months or years go by, it may lose its optimal function of absorbing the heat from the engine. The car mileage may indicate the boiling coolant issue. Using the first-ever coolant, you have beyond 60,000 miles or beyond 30,000 miles after you replace the first coolant, which can lead to a coolant reservoir boiling.

Coolant Flush

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One of the ways to fix the coolant reservoir boiling is to flush the coolant fluid. This is recommended for new cars if the coolant is never replaced when the mileage is 60,000 miles.

In this video, it guides us on a step-by-step on how we can fix the boiling coolant reservoir issue:

  1. Make sure the engine and the cooling system have a cold temperature for you not to burn your skin and for you to be able to touch the system’s surface
  2. Under the car, look for the radiator petcock, which is a draining outlet for the radiator
  3. Unscrew the petcock for the coolant to drain
  4. Use a disposable container to drain the coolant
  5. Fill the empty coolant reservoir with water using a funnel
  6. Open your engine as you finish filling the reservoir with 3 gallons of water
  7. Add in the reservoir the chemical flush, drive around as the water and the chemical flush do their cleansing operation
  8. Let the car cool down, then completely drain all the water from the reservoir through the petcock.
  9. Mix the new coolant (50% antifreeze and 50% water) and add it to the coolant reservoir
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10Add the anti-rust and sealer additive and close the reservoir

Frequently Asked Questions

When Should I Replace A Coolant Reservoir?

It’s time to replace a coolant reservoir is by checking the following:

  • When your car engine keeps on overheating
  • The coolant keeps on leaking 
  • The temperature gauge indicates a very high temperature
  • Sounds of hissing or steam are coming out of the engine

What If The Coolant Reservoir Is Empty?

Without the coolant in the coolant reservoir, the cooling system won’t be able to absorb the heat coming from the engine. Thus, the engine will overheat.

How Is Antifreeze Made?

Antifreeze is made up of ethylene glycol. Ethylene glycol is the product of hydrolysis of ethylene oxide or mixing ethylene oxide with water. 

Ethylene (gas) and water are both colorless, so the color of the antifreeze is from dye formulation.

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