Coolant Reservoir Bubbling: 15 Causes

For anyone who has cooked pasta, the concept of coolant bubbling will be familiar. The creation of air bubbles in both scenarios is driven by the same thing — rising air pressure in a given system.

But while the pasta water is expected to boil (lest you have undercooked puttanesca for dinner), the coolant should never bubble. In fact, it is an anomaly — the causes of which we need to be familiar.

Here are 15 of the most prevalent reasons behind coolant bubbling:

1. Low Coolant Levels
2. Incorrect Coolant
3. Air Pockets
4. Loose or Damaged Radiator Cap
5. Faulty Thermostat
6. Blown Head Gasket
7. Malfunctioning Heater Control Valve or Hose
8. Pollutants and Corrosion
9. Clogged Radiator
10. Failing Cooling Fan
11. Defective Water Pump
12. Turbocharged Engine
13. Oil Contamination
14. Physical Damage to the Coolant Reservoir
15. Overheating Engine

Interestingly, one of the items on the list stems from upgrading the power mill with a turbocharger! That aside, the rest are either component damage or outcomes of driver mispractices and negligence.

Whichever the cause, all of them have equal weight in the propensity of coolant bubbling in a vehicle’s cooling system. Let us delve into these causes individually to better understand how to keep them at bay.

What Is Coolant Bubbling?

Whether you are a nouveau car owner or experienced driver, having questions like “What does bubbling coolant mean?” and “Why is my coolant bubbling?” is totally understandable. After all, it is something we do not encounter every day.

It would be better if things stayed that way. However, we all know that is quite impossible. At some point in our vehicle’s life cycle, its engine temperature will exceed a coolant’s flash point, and coolant reservoir bubbling is bound to occur.

Essentially, coolant bubbling is coolant developing air bubbles as a result of the engine temperature exceeding the coolant’s boiling point mark.

On average, a coolant’s boiling point ranges between 121 °C and 135 °C (250—275 °F, warm-weather coolant) or between 148.9 °C and 204.4 °C (300—400 °F, cold-weather coolant).

These thresholds hold true even if ethylene glycol is added to the mixture, so you should heed them when choosing a coolant variant for your car.

15 Causes of Coolant Reservoir Bubbling

1. Low Coolant Levels

Low coolant levels almost always lead to coolant bubbling, whether due to a coolant leak or insufficiently filling up the coolant reservoir.

There is a good reason why a particular amount of coolant needs to be present in the cooling system — that is, to ensure that heat dissipation occurs accordingly and that the cooling system is not left with more than the operating temperature it can handle.

That said, up-to-spec coolant levels are key to achieving this. Without the right amount of coolant in the system, your car engine will likely overheat, increase operating temperatures, and cause coolant to boil when least expected.

2. Incorrect Coolant

A fact to remember about coolants: the kind of coolant used in a vehicle is always relative to weather or ambient temperatures. Another fact to take note of: not all car owners abide by the rule of thumb I just mentioned.

Coolant is like engine oil in that it requires a certain grade (flash point for antifreeze, to be exact) for specific climatic conditions. Variants used in cold weather typically have a higher flash point to prevent the engine from freezing.

Conversely, those used in warmer climates have a lower flash point to offset exacerbated operating temperatures caused by an easily warmed-up engine.

If these coolant types were interchanged (for instance, a variant with a higher flash point is used in a car driven in Floridian weather), then coolant bubbling is to be expected. That said, this is the 2nd thing you should probably check out if your coolant boils, but your coolant levels are up to spec.

3. Air Pockets

Obstruction to liquid flow is among the first things that could go wrong with your car’s cooling system. Being a closed, pressurized system, it heavily relies on seals to maintain these conditions.

Consequently, it would not operate efficiently — meaning coolant would not circulate seamlessly — if any of its requisites were not met.

When the system is properly sealed, the inside temperatures remain steady, and uninterrupted coolant flow happens as a result. But if one of these seals develops a gap or leak, an opening is created where outside air can enter (hence, the occurrence of air pockets — in turn causing coolant bubbling when revving an engine.

In infrequent and small quantities, air pockets are not necessarily detrimental to the cooling system. But if they remain trapped in the system despite flushing and replacing the coolant, air pockets could indicate pre-existing issues with other cooling system components.

4. Loose or Damaged Radiator Cap

Like seals, the radiator cap serves as a pressure seal and is vital to the overall function of your vehicle’s cooling system. It has to be compatible with the radiator rim, lest it upsets the cooling system’s ideal operating temperature, resulting in coolant bubbling.

Fortunately, an ill-fitted radiator cap is easy to fix — simply ensure that the replacement cap is compatible with the radiator in your engine compartment.

5. Faulty Thermostat

Of the two conditions that need to be met for a cooling system to work efficiently, the thermostat maintains ideal operating temperatures not only of the cooling system but also of the entire internal combustion engine. It does this by letting hot coolant escape the engine and cold coolant enter the engine when necessary via properly working valves.

With a malfunctioning thermostat, the valves do not open and close as described. This, in turn, results in hot coolant remaining in the system when it should not (and vice-versa for cold coolant).

Eventually, the imbalance in the operating temperatures (overly high temperatures, to be exact) causes the coolant to boil.

6. Blown Head Gasket

Although the head gasket’s functions predominantly affect the engine, it is also crucial to the cooling system — in the sense that it keeps the coolant from traveling elsewhere except inside the cooling system.

When a head gasket (view on Amazon) gets busted, the compression gases it is supposed to contain get leaked into the cooling system via the cylinder head. When this happens, coolant bubbling will likely follow suit.

There are many ways to determine if your head gasket is blown or damaged. These indicators are under-hood steam, white smoke from the tailpipe, and overheating (apart from coolant bubbling).

7. Malfunctioning Heater Control Valve or Hose

Faults with the heater core are one of the things that can make your car’s cooling system susceptible to forming air pockets.

Although the component relies on the cooling system to regulate in-cabin temperatures, breaks, or looseness in its hoses or valves, it creates an opening for air to inadvertently enter the cooling system and subsequently cause coolant bubbling.

8. Pollutants and Corrosion

While not exclusive to any system, these two are among the most prolific culprits to just about any vehicular problem (cooling system included).

Rust and contaminants not only defile a car’s overall styling but can also potentially result in a malfunctioning water pump impeller, thermostat (view on Amazon), or radiator.

9. Clogged Radiator

As mentioned earlier, contaminants and rust can lead to an obstructed radiator, which becomes problematic for your cooling system in the long run.

Rust and debris buildup can accumulate in the radiator over time. However, it is preventable if you religiously inspect and clean cooling system components.

Often, accumulation happens in the first place because car owners are inconsistent with the upkeep of their vehicles. You are less likely to encounter this dilemma if inspection and post-ride car are done stringently.

10. Failing Cooling Fan

Other than dirt buildup, components supplementing the radiator can compromise its normal function.

The cooling fan is one thing that comes to mind because when defective, it impedes the radiator from properly dispersing coolant through its fins when excessive heat is generated during the combustion process (causing a slip in the regulation of the cooling system’s operating temperature).

11. Defective Water Pump

The water pump is another vital component of your four-wheeler’s cooling system, with the sole purpose of facilitating coolant circulation from the radiator to the coolant system to the engine and back to the radiator.

It should usher only the coolant and nothing else during this process. However, this might not always be the case — especially if the water pump (view on Amazon) incurs damage.

Many things can cause water pump damage. The main culprits are usually incompatible or low-quality coolant, improper installation, and dry running.

Some of these practices can be hard to overcome, as some car owners do them to save a few hard-earned bucks. However, it would be better to take the brunt of slightly more expensive fluids and component care than suffer costly complications later on.

12. Turbocharged Engine

Having a turbocharged power mill does not necessarily result in coolant bubbling all the time. After all, lots of research and testing goes into retrofitting a vehicle with a turbocharger.

One will need a compatible manifold, exhaust system, oil lubrication, and adjusted compression ratio. But if these prerequisites (among others) are not met, chances are the setup will cause your coolant to boil.

A turbocharged engine entails an increase in compression pressure (sometimes a sharp one at that) to deliver desired power gains. This pressure increase is mostly accounted for in adjustments by experienced car owners or mechanics.

In cases where it is not, however, the resultant temperature rise not only goes beyond the car system’s average threshold but also causes an increase in the coolant system’s operating temperature — the latter being a direct trigger of coolant bubbling.

13. Oil Contamination

Oil getting into the coolant is an aftermath of a bad head gasket or a damaged oil-coolant heat exchanger. Not only that — it is also a huge detriment to the effectiveness of both lubrication and coolant mixture in regulating engine temperature.

One of two things can happen in this scenario. Either the engine oil and coolant are both contaminated with the other substance, or there is only oil contamination in the coolant reservoir but none in the engine oil tank.

If the latter is true, you are experiencing a low-pressure leak. But if it is the former, the contamination is caused by a high-pressure leak (a leak that only occurs between 20 and 25 psi or higher).

14. Physical Damage to the Coolant Reservoir

The coolant reservoir itself (along with its hoses) needs to be stringently and regularly inspected if you want to keep them leak-free and tight. Because if the coolant reservoir or any of its components is compromised, it will fail to mitigate the rising pressure inside the cooling system (you already know what happens next).

Several factors can cause physical damage to the coolant reservoir, including corrosion, age, and natural wear. While all these are gradual processes, they can be exacerbated by engine overheating — an incident that can be attributed to anomalies in multiple car systems.

15. Overheating Engine

Last but not least, we have an overheated engine. And mind you, the power mill only reaches this point when two or more items in this guide have already manifested.

With an overheated engine, coolant bubbling will almost automatically happen since the engine temperature has already gone beyond the coolant’s boiling or flash point. Moreover, engine overheating is never a good sign from a vehicle (especially when it has occurred multiple times).

Despite its thoughtful construction, an engine can only take so much abuse. If a power mill is repeatedly exposed to excessive heat, things would eventually lead to warping or melting of the engine block.

Conclusion — Reasons Behind Coolant Reservoir Bubbling

In conclusion, here are the most common causes of coolant reservoir bubbling:

1. Low Coolant Levels
2. Incorrect Coolant
3. Air Pockets
4. Loose or Damaged Radiator Cap
5. Faulty Thermostat
6. Blown Head Gasket
7. Malfunctioning Heater Control Valve or Hose
8. Pollutants and Corrosion
9. Clogged Radiator
10. Failing Cooling Fan
11. Defective Water Pump
12. Turbocharged Engine
13. Oil Contamination
14. Physical Damage to the Coolant Reservoir
15. Overheating Engine

While some of these items can be easily resolved, anyone would much rather prevent their occurrence. You can, too, by using high-quality, up-to-spec coolant, maintaining OEM-recommended coolant levels, occasionally flushing the coolant, and monitoring the condition of the expansion tank and coolant reservoir.

However, prevention can only take place with the right timing. So if you have missed your chance, go for remedial actions instead.

These fixes start with a proper diagnosis of the problem, which is done by checking coolant levels, examining the integrity of the reservoir and expansion tank, looking for visible holes, cracks, or leaks, and ensuring all connections, valves, and pressure caps are tightly fitted.