Seeing bubbling or boiling coolant in the reservoir can be worrying, but it doesn’t necessarily mean your engine is overheating. The cooling system is a pressurized, sealed environment – so coolant boiling in the reservoir doesn’t suggest the radiator is boiling over. However, the first step should be to stop the car and turn engine off.
A few common causes exist for seeing bubbles or boiling your coolant reservoir, even when the car engine doesn’t overheat. A faulty radiator cap can lower boiling point of coolant and overall system pressure. Combustion gases from a damaged head gasket can cause problems with the cooling system.
Coolant removes the heat from inside the engine to prevent the engine from overheating. So, in our guide, you can learn more about when you notice your coolant boiling but engine not overheating.
By the end, you’ll know the areas to check and how to fix the symptoms causing a lack of engine coolant and cause the engine to overheat. (Read 1999 Ford Ranger 4.0 Camshaft Position Sensor Location)
What Causes the Coolant Reservoir to Bubble and Boil?
There are a few common causes for bubbling or expansion tank boiling radiator coolant: signs of a bad radiator can comprise boiling coolant, steam or smoke pouring from under the hood, and an overheated engine.
A Faulty Radiator Cap
The radiator cap maintains pressure inside the cooling system. A worn-out or faulty cap can allow pressure to escape, lowering the boiling point of the bubbling coolant. This will cause the coolant to boil at a lower coolant temperature than usual. Replacing the radiator cap can restore proper pressure.
A Damaged Head Gasket
A leaking head gasket allows combustion gases to enter the cooling system. This can cause localized boiling, and the coolant expands as the gases bubble through the coolant. A damaged head gasket can be diagnosed through a leak-down or chemical test of the coolant. Replacing the head gasket is the repair.
Air in the Cooling System
If something is letting air into the cooling system, air pockets in the expansion reservoir or expansion tank take up space meant for coolant flow. This reduces efficiency and causes hot spots and boiling coolant. Bleeding the system properly purges air pockets. Coolant system pressure should also be checked.
Incorrectly Mixed Coolant
Using too much water and not enough antifreeze lowers the boiling point temperature of the coolant. This can cause boiling at normal operating temperatures, so draining old coolant every two years and refilling with mixed coolant resolves this.
Why Isn’t the Engine Cooling System Overheating?
If you notice the coolant bubbling in your coolant reservoir or expansion tank, even when the engine isn’t overheating, there are a few reasons this could occur:
The Cooling System is Still Functioning
Even with lower coolant pressure or bubbles in the system, the coolant may flow and properly transfer heat. If the radiator, water pump, thermostat, and radiator fan work correctly, it can compensate for problems like low pressure. (Learn How Many Gallons Of Antifreeze Does A Dodge Ram 1500 Hold)
Heat is Escaping From the Cooling System
With a head gasket leak, combustion gases entering the cooling system create bubbles, but total heat in the system hasn’t increased. The excess exhaust gases escape, preventing a total overheat condition. If you have leaks in the cylinder head area, car coolant leaks spill into other parts of the car’s engine.
The Reservoir Isn’t Representative of the Whole System
The reservoir is only a small part of the entire cooling system. Excess bubbles can accumulate in the reservoir even if most of the system circulates coolant properly. The engine won’t overheat if the engine block can pump coolant uninterrupted.
When to Be Concerned About a Boiling Coolant Reservoir
While a boiling reservoir doesn’t always mean the engine will overheat, it should still be addressed. Here are some signs that point to a more severe problem:
- The temperature gauge is rising or in the red zone
- Low coolant levels in the overflow tank or adding coolant fluid frequently
- The radiator itself is boiling over or spewing coolant
- White exhaust coming from the tailpipe
- Sweet smells coming from the exhaust
- The engine runs hot or stalling
If any of these symptoms occur along with a boiling reservoir, it likely shows a more significant cooling system problem.
The most common causes are faulty thermostat sticking, a bad water pump, cooling fan issues, or a severely low coolant level. These conditions will lead to eventual overheating, and the heat from the engine will cause damage if not repaired.
Replacing the thermostat can be the first step to make the coolant pass around the engine to the radiator, where it can be cooled.
How to Diagnose the Cause of a Boiling Coolant Reservoir
To get to the root of the problem, you or your mechanic can perform some diagnostic steps:
- Check the coolant level – Low coolant can cause pockets of air that lead to boiling. Top off if needed.
- Test pressure in the cooling system – This can identify leaking components or a bad radiator cap. Normal pressure is around 15 PSI in this type of sealed system.
- Combustion gas test – This checks for exhaust gases in the coolant from head gasket failure.
- Dye test – Adding fluorescent dye to the coolant can help spot external leaks from hoses, gaskets, etc.
- Flow test – Verify coolant is circulating through the radiator properly. A slow flow prevents the coolant from reaching the radiator quickly, where the coolant gets too hot.
- Boiling point test – Determine if the freezing and boiling points of the coolant mixture are in spec.
The correct repairs can be made once the root cause is found through testing. This will stop coolant reservoir from boiling, thus removing heat from the car engine to prevent the engine overheating.
How to Stop Coolant System Reservoir Reaching Boiling Point
Here are some tips to stop coolant reservoir bubbles and boiling:
- Replace the radiator cap if it is old or faulty to maintain system pressure.
- Bleed air from the cooling system and top off the coolant level. Ensure to use the recommended 50/50 antifreeze from the engine manufacturer or car manufacturer.
- Replace the leaky head gasket if it is leaking and allowing exhaust gases to enter.
- Do a coolant flush and then refill with fresh coolant to prevent contamination.
- Check radiator hoses for soft, swollen spots indicating internal degradation. Replace any suspect hoses.
- Ensure electric cooling fans operate properly and turn on at the correct temperature.
- Inspect the water pump for leaks and ensure it spins freely. Replace if necessary.
- Verify the thermostat opens at the proper temperature to allow coolant flow to the radiator.
Following a complete cooling system inspection and making necessary repairs will get to the bottom of that boiling coolant reservoir. That will restore proper operation and prevent engine overheating worries. (Read Why Is My Radiator Empty But Reservoir Full)
Conclusion: Coolant Reservoir Boiling But Engine Running?
Bubbling or boiling coolant in the reservoir can signal underlying issues that need attention. While the engine may not be overheating, it’s best to identify and address the root cause.
Begin with a cooling system inspection and pressure test. Flush old coolant, bleed out air pockets, and fix any leaks or bad parts.
If you can’t diagnose the specific problem, take your car to a professional mechanic or dealership service department. They have the tools and expertise to test the cooling system thoroughly.
FAQs Car Coolant Boiling But Radiator Doesn’t Overheat
What should I do if my coolant reservoir is boiling, but the engine isn’t overheating?
Do not ignore a boiling coolant reservoir, even if the engine isn’t overheating. Have the cooling system tested to determine the cause – it could be something minor, like a low coolant level or a faulty radiator cap. But it could also suggest problems like a leaking head gasket or a bad water pump.
Is it safe to drive with a boiling coolant reservoir?
It’s not recommended to drive with a boiling coolant reservoir. While the engine may not be overheating, it shows an issue with system pressure, coolant level, or circulation. Driving might allow conditions to worsen and lead to eventual overheating, which can cause severe engine damage.
Why does my car’s coolant reservoir bubble after shutting off a hot engine?
This usually occurs from normal thermal expansion as the engine heat dissipates. Hot coolant in the system won’t provide enough air circulation when your car stops after shutdown and will start boiling in hot areas. The coolant then expands back into the reservoir as the remaining heat escapes. This process can cause some harmless bubbling in the reservoir.
Can a blown head gasket cause bubbles in the coolant reservoir?
Yes, a blown head gasket is a common cause of bubbling or boiling in the coolant reservoir. It allows exhaust gases to get into the cooling system, and those gases will bubble through the coolant. White exhaust smoke, sweet coolant smells, overheating, and boiling reservoir suggest a bad head gasket. (Learn How To Flush Milky Oil From Engine)
What could cause my car’s temperature gauge to stay normal even if the coolant is boiling?
The coolant could boil only in a localized spot, like the reservoir, but overall coolant circulation continues. Air pockets, a weak coolant mixture, or minor leaks may allow boiling without system-wide overheating. The temperature sensor could also malfunction and fail to register an overheating condition.
