What to do if the fuel oil in the expansion tank is boiling?

Why the antifreeze may boil in the expansion tank and how to correctly act in such a situation

Motorists who care about the integrity and safety of their car, are required to know about the peculiarities of the cooling system. One of their characteristic troubles is boiling of cooling liquid.

Antifreeze expansion tank

As coolant, depending on the car, antifreeze or toluene acts. The fluids are virtually the same, only different manufacturers use their own formulations, additives and additives, which somewhat changes the properties of different mixtures.

The fluid that provides engine cooling and the work of the heating system for heating the passenger compartment, may boil for several reasons. And it is an extremely undesirable phenomenon, as it leads to serious negative consequences.

To prevent such situations, you should know why the coolant boils, why the temperature rises to the limits, and how to act correctly in the case of boiling of toluene or antifreeze.

Causes of boiling

The first thing to do is to understand why the antifreeze in the expansion tank may boil and what nodes are to blame.

Antifreeze reservoir

Understanding the essence of what is happening, the car owner will be easier to take timely action, replace problematic components and prevent dangerous consequences.

Existing actual causes, due to which the coolant boils in the expansion tank, whether it is antifreeze or tosol, are associated with 3 elements:

  • The water pump;
  • radiator;
  • thermostat.

A little later, you will learn what to do if you suddenly notice your car is boiling with the coolant. Obviously, it is not necessary to continue driving the car, as well as to do a number of other manipulations.

In addition to the above devices, the brackish can also boil because of the failed fan and an extremely trivial but common cause. It consists in a shortage of cooling fluid, which can be checked with the help of an expansion tank. There are special marks on the tank, which allow you to understand how much the coolant level is less than required.

Therefore firstly try to add missing amount of antifreeze. If it is not in the level, then look for other reasons, ruling out possible malfunctions and eliminating the identified problems.

The main task of a car owner is to eliminate possible provocateurs of boiling coolant. Each of the nodes should be examined separately.


To pump fluid through the engine cooling system, there is a special pump or water pump.

Passat pump pulley

The device can lose its efficiency, begin to poorly pump coolant through the system, as a result of which the antifreeze overheats and begins to boil.

Such a malfunction occurs mainly on cars where the water pump pulley performs its rotation at the expense of a separate belt in the timing system. Or simply the timing system.

To further avoid overheating of antifreeze, excluding its possible boiling, the task of the motorist is to periodically check the degree of belt tension. If it becomes loose, it will be necessary to make the appropriate adjustment and tightening. If completely worn, the belt must be replaced.

It’s when the tension loosens that the velocity of the antifreeze moves through the system decreases. This causes the antifreeze to remove less heat from the engine. This is how the water pump provokes the cooling liquid to boil.

The radiator

Another device, due to which the coolant may boil, is the radiator of the cooling system.

The radiator in the car

At the same time, the radiator itself fails for several different reasons, for the elimination of which appropriate measures should be taken.

  1. Scale formation on the inner walls. It is connected with long-term operation, usage of low-quality cooling fluids, as well as with filling the system with plain water instead of a special coolant. Scale is accumulated gradually, and with a slight contamination problems with antifreeze are not observed. When it becomes a lot of scale, it reduces the heat conductivity of radiator tubes. As a result, the motorist is faced with a situation where the coolant is not effectively cooled in the radiator and poorly dissipates heat. This is when he notices that the fluid is boiling.
  2. The radiator may be clogged with debris, which gets inside the element together with antifreeze. Dirt and debris can appear there due to improperly performed replacement procedure, too old coolant or as a result of mixing incompatible fluids. Contaminants contribute to the reduction of radiator capacity, and therefore the efficiency of the whole cooling system decreases.
  3. Prolonged driving at low speed. A frequent problem for those who drive off-road or have to constantly stand in traffic jams. Cars are provided with cooling by counter air flows. In traffic jams and off-road, when the speed is low, the radiator can not cool completely. That’s when the fan comes on. Sometimes the radiator fan fails, doesn’t spin fast enough, or stops working altogether.

External contaminants also contribute to reducing the efficiency of the radiator, so it should be periodically cleaned and monitored.


Possible causes of coolant boiling should definitely include the thermostat, which is designed to control the temperature of the cooling fluid. Depending on the indicators, the thermostat starts the coolant in a small or large circle of circulation.

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Coolant temperature gauge

When the thermostat fails, the special valves included in its design, jam in one unchanging position. Because of this, the coolant only flows in a small circle, bypassing the engine cooling system radiator.

Such limited circulation quickly leads to the fact that the coolant overheats and reaches its peak boiling point. If the antifreeze is boiling, it is better to unscrew the cap on the expansion tank, thus releasing the extraneous steam.

In the underhood space to the thermostat lead two branch pipes. This is where the coolant flows through the device, goes to the radiator and back. The thermostat has thermostat valves that are triggered at a certain temperature. Touch the antifreeze inlet hose that carries the coolant to the radiator and compare its temperature to the return pipe. If the temperature of the former is higher than the latter, the thermostat is jammed and needs to be replaced.

Possible Consequences

A high level of heating of the antifreeze or heating water is a normal phenomenon for a running internal combustion engine. The only question is how much coolant warms up, and whether the temperature does not go beyond the set boiling point.

Front crankshaft oil seal

It is impossible to say for sure at what temperature the antifreeze or the antifreeze will boil. It all depends on the specific manufacturer, who adds certain additives to the composition.

In fact, antifreeze is purified water with special additives. Just they largely determine the quality and the temperature stability of the liquid. At that, coolant may be of different colors. The dye itself does not influence the properties, but allows dividing the antifreezes and antifreeze and toluene into several categories. Some of them may be mixed, and others categorically may not be added to one expansion tank at the same time.

If we talk about how many degrees it will start boiling, then you need to proceed from the physical and chemical properties of the fluid and additives used. Earlier boiling occurred as in the case of ordinary water, i.e. at 100 degrees Celsius. But in modern antifreezes the temperature border of boiling is bigger, and as a result coolant boils over at about 101-108 degrees Celsius.

It is difficult to determine the exact boiling point to within a degree. It depends on the specific manufacturer and its peculiarities of manufacturing the cooling fluid. It is recommended to start with your own coolant used in your car. That’s how you’ll know if the coolant boils in the tank at 100 degrees Celsius or more.

Boiling is indicated by the obvious signs when the brine is literally beating out from under the cap. This tells the motorist that the coolant is boiling. And therefore, it is necessary to take appropriate measures in order to avoid undesirable consequences.

Some motorists underestimate the degree of danger of boiling and bubbling antifreeze. As a result, they do not pay attention to it, continue driving and think that they will solve the problem when they get home, garage or nearest automobile service center on their own way.

But if you do so, you can provoke serious trouble and cause dangerous damage to the engine and related systems. The consequences of boiling depend on how strong overheating of the engine will be because of boiling coolant. Three stages are distinguished:

Let’s start with the safest mild overheating of the internal combustion engine. If from the beginning of boiling coolant has passed literally 10 minutes, but no more, problems are practically excluded, that is, there is no serious threat to the integrity of the motor.

It happens not seldom, because not all drivers immediately notice the signs of boiling. Often the corresponding sensor on the dashboard will tell about exceeding the temperature limit of the engine. If the sensor gave a signal, it is recommended to turn off the motor immediately. Failure to do so may cause the motor pistons to start melting.

There is nothing scary or dangerous about mild overheating in modern vehicles. It is only important to stop in time, so that overheating does not start to turn into the medium and strong forms.

The average level of engine overheating, caused by boiling of antifreeze or toluene in the system, comes next. This is a more serious situation, which can lead to the following consequences:

  • formation of cracks and deformation of the cylinder head;
  • Creation of oil leaks through the oil seals;
  • Damage of piston ring baffles;
  • Burning of the cylinder-head gasket.

Read also: What should be the pressure in the cooling system, how it is created, and what to do if there are deviations from the norm

This happens when the coolant keeps boiling for more than 20 minutes.

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Medium and weak overheating is mainly encountered by those motorists who are negligent to the issue of cooling system maintenance, forget to add antifreeze in time and do not change the coolant according to the recommendations.

The most undesirable is rightly considered to be severe overheating. If it occurs, the car owner risks to lose the engine, which will need overhaul or deserved sending to scrap.

If the engine temperature has been over the red line for a long time, literally all components of the internal combustion engine will suffer. But even this is not the most terrible scenario. In practice, there were cases when prolonged severe overheating became the cause of engine explosion.

But the situation comes to severe overheating very seldom. Even at the initial stages, most drivers manage to shut down the engine and take a number of other measures aimed at engine safety and their own safety.

In fact, the strong overheating is followed by a wave of destructive phenomena. Pistons are the first to burn, then the melted metal appears on the piston walls, pistons themselves fail, the cylinder block is deformed. Do not forget about the destruction of the lubrication system, crankshaft, etc. Eventually, the piston may pierce the wall of the block, and the engine is unlikely to be restored to its former state.

Driver Actions

Noticing that the tonol starts to boil in the expansion tank, it is necessary to take a number of obligatory measures. Do not wait for the coolant to start boiling over a hot engine. Since the fluid is boiling, there is a proper reason for it.

Car antifreeze

You already know at what temperature the antifreeze in cars usually boils, what provokes overheating and to what consequences it can lead. Therefore, no one will want to face such a situation on your own car. It is necessary to start cooling the engine by available means, not forgetting that the coolant is under high pressure now.

There are some useful tips and a certain algorithm of actions, allowing even a beginner to understand what to do, if suddenly the fuel oil boiled.

  1. Be sure to try to quickly unload the engine. To do this, you can shift the gearbox into neutral and drive to a full stop.
  2. Simultaneously, switch on the fan and heater. Even if it is hot outside, the heater will take some of the heat, thus you will speed up the process of engine’s temperature fall.
  3. After stopping the car, switch off the engine. While doing so, leave the heater running.
  4. Pull the handle or press the button to open the hood from the cabin and get to the underhood space. There is no need to stand in front of the hood and open it, because hot steam will immediately come out of there.
  5. By opening the hood, the steam will come out and fresh air will begin to come to the motor. Wait a while. Then lift the hood and lock it in place. It is best to have gloves or a rag with you so that you do not grasp the possibly red-hot elements with your bare hands.
  6. Do not try to open the cap of the expansion tank right away. Otherwise you risk your own health. The pressurized liquid even with a small opening of the cap may spurt out and get on your skin.
  7. Wait about 30 minutes. During this time, you can try to find a driver to get you to a service station, or call a tow truck service.
  8. When the engine cools down a bit, open the expansion tank cap. Surely the coolant has become much less than the proper level. Fill the tank with fluid in small portions. The heater is still running.
  9. Do not immediately pour in a large volume of coolant if its level after boiling has become minimal. Abrupt mixing of cold antifreeze or coolant with hot coolant can provoke dangerous negative consequences.
  10. If you can only get to a service station on your own, after refilling the coolant and cooling down, start the engine and drive it, taking care not to overload it. When the temperature again reaches over 100 degrees, stop, turn off the engine, and again wait until it cools down.

Also do not sit and do nothing. If the coolant continues to boil, it will overheat the engine first to a medium, and then to a strong level. You already know about the consequences of such heating.

On the basis of all considered above, it is possible to speak with confidence about the seriousness of the problem of boiling coolant. To prevent such situations, closely monitor the state of the cooling system, check the performance of the main nodes and try to carry out timely maintenance.

Boiling Desaline

Why does the ash is boiling? Such a situation may arise for a variety of reasons, for example, there was a depressurization of the cap of the expansion tank of the cooling system, thermostat malfunction, lowered the level of coolant, bad was poured, out of order of the cooling fan or temperature sensor. The main thing the driver of a car with boiling antifreeze must remember is that further movement is impossible! Failure to follow this rule can lead to complete engine failure, which is fraught with expensive and complicated repairs. However, to eliminate the causes of boiling antifreeze is really not that difficult, and sometimes even a novice car owner can do it.

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The drip of ash

Reasons of boiling and their solution

To begin with, let’s analyze in detail all the reasons why the brine boils.

    Faulty thermostat . The main task of this device is not to supply coolant to the radiator until the engine has not gained a certain operating temperature (usually +85 ° C), that is to transfer it to the so-called “big circle”. However, if the unit will not turn on in time and does not circulate the coolant through the system, it will quickly heat up in the “small circle” with the engine and simply boil, because it will not have time to cool.

A pump ruptured with tosol

To restore serviceability of cooling system, and in future to prevent a situation, when quickly boils tosol or antifreeze, it is necessary to perform revision of the listed above nodes. Let’s list, in what sequence it is necessary to check the above nodes in accordance with the probability and frequency of their failure.

  1. The expansion tank and its cover . It is especially urgent for the cases, if the antifreeze boiled in the expansion tank, and steam is coming out from under it. It is better to replace the cap with the valve as a whole.
  2. Thermostat . This unit should be checked if the radiator is cold when the engine is on, and the coolant is boiling. It also makes sense to check the thermostat after replacing the coolant if it immediately boils.
  3. Cooling fan . It does not often fail, but it makes sense to check it. As a rule, problems are caused by loose contacts or breakdown of insulation of stator and/or rotor windings.
  4. Temperature sensor . The device is reliable enough, but on older machines sometimes fails. Actually it controls the work of the fan on the radiator
  5. Centrifugal pump (pump). Here is similar to the previous item.
  6. Cooling radiator . It is necessary to carefully inspect it for damage and possible coolant leaks. If it is leaking (this will be accompanied by a situation when antifreeze escapes), it is necessary to dismantle and solder it. As a last resort replace it with a new one. You can also just clean it if it is very clogged. For external cleaning it is better to remove it. And internal cleaning is done together with the whole cooling system (without disassembly).
  7. Check the level of antifreeze in the system. It can leak out of a damaged system, and the remaining volume can not withstand the heat load and boil. If low quality fluid with a low boiling point is used, it must be replaced completely. Otherwise, you can simply refill the antifreeze.
  8. Check whether the filled antifreeze is suitable for the current machine . If two types of coolant have been mixed, check that they are compatible with each other.
  9. Check the operation of the safety release valve . Check the operation of the valve on the cap by means of a polyethylene sheet.
  10. Check the quality of the filled antifreeze . This can be done by several methods, using both professional equipment and improvised means available in the garage or at home.

Bad antifreeze is the death to the engine

The boiling and freezing point of the coolant depends on its composition. The consequence of pouring bad (fake) tosol – boiling and destruction of the engine. Learn the signs and methods of testing to determine if it is bad More

As a rule, it is necessary to perform only one of the listed items. However in complex situations it is possible that more than one of the listed items may fail.

Remember that all repair and preventive maintenance work on the cooling system must be done only when the engine is cold. In no case you must not open the cap of an expansion tank when the engine is hot! So you risk to receive strong burn!

Boiling often occurs when driving in a lower gear with the engine at high revolutions, for example, when driving in the mountains for a long time or in city traffic jams in the summer heat. Situation is aggravated, if the conditioner is switched on, as it gives additional loading on cooling system, in particular, on the basic radiator. Therefore before a trip to mountains always check the condition of engine cooling system, including its antifreeze level. If necessary top it up or replace it.

Do not use antifreeze containing more than 60% of ethylene glycol and less than 40% of water.

Not infrequently the cause of boiling antifreeze can be air lock in the cooling system. Its symptoms are thermostat problems, antifreeze leakage, problems with the pump and cabin heater. Therefore, if at least one of the above problems is on your car, it is recommended to fix the situation, because ignoring it can also provoke the boiling of the motor.

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Some drivers are interested in the question of why the antifreeze boils after a stop? There are several possible variants. The first is when the car is standing with the engine running. So, it is just a coincidence, and you are lucky that you found the occurrence of the situation, when the antifreeze boiled not in motion, but on the road or in the garage. In this case, immediately turn off the engine and set the car on the handbrake. On further actions we will talk a little later.

Low antifreeze level

Another option is that the smoke (steam) continues to come from under the hood even after you have detected boiling and stopped at the curb. You need to understand that most fluids, and antifreeze is no exception, has a high heat capacity. And it means it takes quite a long time to heat up and cool down. Therefore, on the face of it is a situation when you observe the boiling coolant, which will stop evaporating after some time after the engine is stopped.

There are exotic variants, when there is boiling in the expansion tank after the engine is stopped. For example, the situation described below is relevant for the car Chrysler Stratus. It consists in the fact that after the engine is switched off, the radiator safety-relief valve releases the pressure into an expansion tank. And there is an effect that everything boils there. Many motorists take this process as a puncture of the cylinder-head gasket and rush to change it. However, there is no need to rush, and instead it makes sense to carefully study the scheme of the cooling system of a particular car.

What are the consequences when the antifreeze boiled over?

The consequences of the fact that the antifreeze boiled over depend on how much the engine overheated. This in its turn depends on the car brand (engine power and body weight), engine construction and the time between overheating and engine shutdown (the moment when it stopped and started to cool down). Let’s divide conditionally possible consequences into three degrees – light, average and heavy.

So, at light overheating of the engine (up to 10 minutes) a slight melting of the engine pistons is possible. At the same time, they may slightly change their geometry. In most cases, this situation is not critical, unless there were problems with geometry before. If you notice boiling of the drip and take appropriate measures, which will be discussed below, it is enough to eliminate the cause of the breakdown and everything will be all right.

To find out the cause of engine overheating it is necessary to check whether the radiator is not clogged, also. Read more

The average case of overheating occurs about 20 minutes after the boiling of the saline or antifreeze. Thus, the following types of breakdowns are possible:

  • Warping of the cylinder head housing (relevant when the engine temperature reaches +120 degrees or higher);
  • cracks (both micro cracks and cracks visible to the human eye) can appear on the cylinder head;
  • melting or burning of the cylinder block gasket;
  • Failure (usually complete destruction) of inter-ring dividers that stand on the engine pistons;
  • oil seals will start leaking engine oil, and it can both flow out and mix with boiling antifreeze.

Already listed breakages are enough to imagine the scale of the tragedy, which can happen with the car in case of boiled antifreeze. All this is fraught with major engine repairs.

Expansion tank with a lid

However, if the driver, for whatever reason, ignored the boil and continues to drive on, then the so-called critical “wave of destruction” occurs. In very rare cases the engine can simply explode, that is completely burst and fail, but it does not happen very often. As a rule, destruction occurs in the following sequence:

  1. Melting and combustion of the engine pistons.
  2. During this melting process, the molten metal gets on the cylinder walls, thereby making it difficult for the pistons to move. Eventually, the piston is also destroyed.
  3. Often, after the pistons fail, the car will simply stop and stall. However, if that doesn’t happen, then engine oil problems begin.
  4. Due to the fact that the oil also gains critical temperature, it loses its operational properties, due to which all rubbing parts of the engine are put under attack.
  5. Usually small parts melt, and in liquid form they stick to the crankshaft, which naturally makes it difficult to rotate.
  6. After that, the valve seats begin to fly out. This causes at least one piston to cause the crankshaft to simply break, or at the very least, to bend.
  7. The broken shaft can easily pierce one of the walls of the cylinder block, and this already amounts to total failure of the engine, and the most interesting that such engine can hardly be restored.
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Obviously, the consequences of boiling of ash in a cooling system can be very sad for a car and its owner as well. Accordingly, it is necessary to keep the cooling system in order, regularly monitor the antifreeze level, and if necessary, top it up to the normal level. And in the case when the boiling still happened, it is necessary to react as soon as possible, and take actions to fix the problem.

What to do if the antifreeze boils

What to do if the engine boiled

However the most interesting and interesting for drivers is the following question – what to do if the antifreeze/thickness boiled on the road or at a parking lot. The first thing you should remember is not to panic, i.e. to keep the situation under control! It is desirable to pay attention to the fact, that the cooling system is partially out of order as early as possible. It is possible to do it with the help of devices on the panel, as well as visually by steam coming out from under the hood. The earlier you take appropriate actions – the more probability to get off with inexpensive repair.

There is a simple algorithm, which every motorist should know, even the one who has never faced such a situation. It consists of the following steps:

  1. Switch to neutral speed and reset the engine speed to idle.
  2. Continue driving, and do not drop speed sharply. The ambient air will blow around the engine to cool it down as much as possible.
  3. Also on the run switch on the cabin heater, at the maximum possible temperature. And it is necessary to do it irrespective of a season, that is, if necessary, even in summer heat. This procedure is done in order to remove heat from the radiator as much as possible and it cools down as much as possible even at speed without load.
  4. It is necessary to drive as long as possible, to a full stop (if it happens in summer, it is desirable to find a place to stop somewhere in the shade, without direct sunlight). After the engine must be turned off. At the same time the ignition must be left on in order to let the heater work for another 5 … 10 minutes. Then turn off the ignition.
  5. Open the hood in order to give maximum access of natural air into the underhood space. Without touching any parts of the engine with your hands (now they have an extremely high temperature) wait for a certain time. In summer it is about 40 … 50 minutes, in winter about 20 minutes. Depends on the weather conditions and the time while the car was “boiling”.
  6. Call a tow truck or a car that will tow the car to a service station or a good technician with the appropriate diagnostic equipment.

Algorithm of actions is not complicated, and even inexperienced driver can cope with it. The main thing is to notice in time the process of boiling of antifreeze. And it is always desirable to have in a trunk a small stock of coolant (similar or compatible with used at the moment), as well as engine oil. A canister doesn’t take much space, but may come in handy at the critical moment.

What not to do when the engine is boiling

There are a number of strict rules that limit the actions of the driver during the occurrence of a situation when the antifreeze boils in the radiator, expansion tank or other element of the cooling system. These rules are designed to protect human health from serious injury, as well as to minimize material losses that may arise in the described situation.

  1. Do not load the engine (do not gas up, and instead you need to reduce the revolutions to the idle value, usually around 1000 rpm).
  2. Do not stop and turn off the engine thinking that the engine will stop boiling.
  3. Do not touch the heated underhood parts!
  4. While there is steam coming out from under the cap of an expansion tank or other unit and while the bubbling of the brine in the system it is categorically impossible to open the cap of the expansion tank! This may only be done after the time specified above.
  5. Do not pour cold water on the engine! It is necessary to wait for the engine to cool down by itself.
  6. After the engine has cooled down and the new antifreeze has been added, you must not drive after reaching a temperature of more than +90 degrees.

Compliance with these simple rules will ensure the safety of the driver, as well as minimize the degree of failure and, consequently, possible material costs.

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