Cars utilize either a liquid or air-cooling system to keep the engine running optimally, but most vehicles rely on the liquid cooling system. Keep reading for everything you need to know. Cars depend on a tightly knit network of pipes to deliver the coolant to the engine.
This coolant is contained in a rubber tank known as the coolant reservoir; from there; it sends out coolant to engine parts such as the cylinder head.
But at one point, you notice your coolant reservoir is boiling, and you wonder why?
Bubbles in a coolant reservoir could serve a couple of purposes. Some won’t even have you lose a wink of sleep over, otherwise, you need to start taking action right away.
What causes bubbles in the coolant reservoir?
Bubble Up! The air in the coolant reservoir, how and why?
If you notice some bubbles forming in your reservoir tank, don’t panic! Many vehicles’ cooling system is pressurized and depend on a closed hose system for the flow of liquid around the engine.
When and if air gets into the closed system there forms an air pocket forms that shuts off the flow of liquid, causing a rise in the temperature until the coolant boils in the reservoir.
Bubbles churning in reservoirs could be a serious matter, and sometimes it’s a nothing-to-worry-about deal because some actions will naturally sip air into the cooling system.
However, in other instances, the air in the cooling system spells some defective component needing attention. Let’s have a look at these.
Air pockets
When coolant is flushed and refilled, unfilled pockets of air are trapped in the cooling system. As you drive the car and it heats, the air pockets expand and are forced into the coolant reservoir.
When the engine cools down, the expanded air pockets contract; hence, coolant is sucked from the reservoir to replace air. This causes the bubbles. This, of course, is all normal, but it can be avoided.
Faulty radiator cap
The radiator cap simply works as a pressure seal to keep pressure in the cooling system high, while allowing coolant to flow to the expansion tank. If this is bad, or is changed to one that is not compatible, air is allowed into the cooling system, and the coolant reservoir will boil.
A quick (and dirty!) way of knowing that bubbling is caused by this is the fact that you will see bubbles in the coolant reservoir after driving for a long time.
Bad thermostat
The thermostat regulates the flow of coolant in and out of the radiator. Here, the thermostat must open and close at a specific time.
It must open so that hot coolant from the engine flows out and into the radiator, where it cools off. Then, the thermostat, to start the car, remain close for the engine to heat the coolant.
If the thermostat is bad, it doesn’t perform this function as it should, of opening and closing at the proper time. This causes the bubbling effect in the coolant reservoir/radiator since air flows in uncontrolled.
Leaky or blown head gasket
A leaky or blown head gasket is a common cause of bubbles from the tank. With the head gasket blown, the gases of combustion from either of the cylinders escape into the water jacket and from there pass into the rest of the cooling system through the cylinder head.
One easy way to ascertain that this air comes from a leaking gasket is to notice that it bubbles out immediately after the engine is started.
Faulty heater control valve or hose
The heater control allows hot anti-freeze to enter the heater’s core and help keep the passenger compartment warm.
When this valve goes wrong, or the heater’s hose connection is not good and taut, air slips into the cooling system, which causes the anti-freeze to boil in the reservoir.
Leaky coolant reservoir hose
The hose that connects the radiator to the coolant reservoir is “all-important. It MUST be leak-proof and tight.
If the radiator hose goes bad or is worn and spongy, it allows air to sip into the hose, and this again causes the anti-freeze to boil in the coolant tank.
Faulty water pump
The water pump helps in the circulation of antifreeze through the cooling tubes in the radiator. Having cooled in the radiator, and flowing back through the water pump, it winds its way there through all the various paths and pipes leading to the various live parts of the engine.
With a bad water pump, air is allowed to enter through the pump intake. This air when it reaches the radiator intake or the valve or cooler, causes the bubbling we see in the anti-freeze.
Rust and contamination
Like other metal parts, a radiator can rust in time. If a radiator is not flushed and kept supplied with anti-freeze, it will soon collect impurities.
Rust, grime, and other substances may block the radiator’s tubes and tend to interfere with the perfect performance of the thermostat and water pump.
Then the anti-freeze boils from excess of heat and gives rise to the quickly appearing bubbles we see in the coolant reservoir.
How do you fix the air bubble in your coolant?
As a general rule, one way to fix a problem is always to attack the original cause of the problem. So the question is, what do I do if my coolant reservoir is bubbling? Let’s find out.
Let the air leave the post refill
When flushing and filling-up the coolant reservoir/cooling-radiator, air gets trapped in the system. So ensure air leaves it as well. Start your engine and let it run for fifteen mins with the rad’s cap off. The air should bleed off in the radiator or reservoir before you put the cover back on top of it.
Replace the faulty thermostat
With a faulty thermostat, the coolant does not flow well. This causes boiling, and the best way to fix it is to get it changed to an OEM or compatible aftermarket part. So long as the thermostat isn’t busted, coolant will move about well enough, and there won’t be bubbling.
Fix a leaky head gasket
Since combustion gas infiltrates the cooling-system as a result of the leak in the head gasket, one has to seal it. That can easily be done.
Take the car to the professionals and let them fix it. If you are keen on saving some bucks, you can use a leak repair fluid to fix it. However, if the leak refuses to budge, the safest fix is a change of the head gasket.
As you may agree, head gaskets are fixed in places where to get to many, nearly half of the engine has to be put upside down, so you cannot kid yourself about replacing it. This is one reason you don’t want the head gasket to go that way. And as if that isn’t enough, if you decide not to fix it, permanent engine damage occurs.
Replace or seal the rad cap
Letting the cap off the coolant and air pressure begins fluctuating, damagingly, and allowing air into the cooling-system, you will need to change the rad-cap. It is safe to replace it with an OEM or manufacturer-recommended cap.
If an incompatible redcap is put there, air will get into the cooling system too.
Replace or seal the coolant reservoir hose.
Reservoir hose leaking
What you don’t want gets in, and what you need to have goes out. So, it is a wily way of yours to replace the hose quickly.
Because, aside from air that gets in the cooling system through the leaky hose, coolant drops from the leaky hose. So, you still get to have low coolant levels, causing the engine to overheat.
Replace the heater control valve and hose
Aside from air that leaks into the cooling system. A vehicle occupant also suffers from it. You should then replace the heater valve and tighten the hose. If the hose is worn, you still have to change it.
Change the bad water pump
The water pump is designed to remain closed till the cap is defective and traps air in the pump intake. The wise thing to do here will be to replace this cap. With the cap closed, no air is trapped inside the water pump.
Flush the coolant reservoir and radiator
As coolants do work to a level, they become dirty, forming sludge and grime, and are filled with other particles.
The radiator then gets clogged, not allowing other connected components to function well. Only regularly do ensure to the flush the radiator or coolant reservoir and refill it with new coolant.
FAQs.
Q: Is it normal for the coolant reservoir to bubble?
Bubbling in a coolant reservoir sometimes is normal. An example of this would be a coolant reservoir bubbling when the car is off. How does that happen? All cooling systems will stop working if you park your vehicle and turn off the engine. But the engine is still hot.
The water pump, cooling fan, and every other cooling component have stopped working. But coolant is still circulating in the engine. The heat that builds up in the engine with nothing to cool it down at this moment is the reason your reservoir is bubbling. But that is only temporary, so nothing to worry about.
Q: Can a bad thermostat cause bubbling in the coolant reservoir?
A bad thermostat can cause bubbling in the coolant reservoir. The thermostat ensures the coolant stays where it is needed and leaves when necessary. But when it goes bad, there will be an unregulated flow of coolant and bubbles in the reservoir.
Q: Can a bad water pump cause coolant to bubble?
Yes, a bad water pump can cause coolant to bubble. The water pump helps circulate the coolant through the radiator tubes, where it cools. The cool coolant then spreads into the engine components through the engine’s pipes or passages.
With the pump bad, there won’t be anything to circulate it to the radiator. And first, the engine component won’t get enough coolant to lubricate properly, and so won’t function well. Second, since the circulating coolant doesn’t get to cool, it will cause the engine to overheat and cause a bubble.
Q: Can an air bubble cause a car to overheat?
Air bubbles can make your car overheat, depending on where this bubble finds itself trapped. Naturally, you will find bubbles in the coolant reservoir. But bubbles in the reservoir make the rate at which the coolant absorbs heat quite low.
Since coolant doesn’t absorb the heat of the engine at the right time, the engine builds up heat. This build-up of heat that isn’t timely cooled causes the engine to overheat.
If this air happens to be in the radiator, it prevents coolant from appropriately circulating in the engine. So long as the engine doesn’t get coolant to cool it when hot, it overheats.
Why is my coolant boiling, but the car is not overheating?
Sometimes, coolant bubbling in the reservoir but the engine not overheating, is normal. Here, it’s most probable that what your car needs might be a light repair. It could be as a result of contaminated/low coolant or a bad air radiator cap. You may need a proper diagnosis to reveal the root cause.
Try replacing the radiator cap. You can also top the coolant or complete a coolant flush, then replace it. Drive the car if the bubbling stops, the cause is a faulty radiator or contaminated/low coolant. If the bubble still occurs, take the vehicle to a mechanic.
Final words
Bubbles in a coolant reservoir could mean something serious, and at the same time, it could be something not to worry about.
Why? The reservoir can boil after you turn off your car; the reason is with the car off, the cooling system shuts down, but the engine is still hot! Even air can be trapped in the cooling system after getting a coolant flush.
On the other hand, faulty engine or cooling system components can let air into the cooling system. Check the head gasket, radiator cap, and other parts yourself, or have a mechanic do it. And fix whatever is causing bubbles in the reservoir before it leads to catastrophic engine damage!