Why is the coolant boiling in the car?

There a variety of reasons which lead to a boiled coolant, for example, the cover of the coolant tank has been depressurized, the thermostat is bad or failing, the coolant level has dropped, bad quality of the coolant or wrong coolant type, the cooling fan or temperature sensor has failed. The main thing that the driver with boiling antifreeze should remember is that you need to stop the car right away if you notice this problem.

If you continue to drive like this, it will lead to complete engine failure, which is followed by expensive and difficult repairs. However, it is actually not so difficult to eliminate the causes of why the coolant is boiling, and sometimes even a novice car owner can do it.

What are the causes of boiling coolant?

First of all we will begin by talking a bit about all the possible causes of why the coolant is boiling

1. Bad radiator

The function of this car part is to cool down the coolant and keep the cooling system in good working condition. However, it can get mechanical damage or simply clog inside or outside. It’s one of the most common causes, so it’s better to check the radiator first.

2. Bad or failing thermostat

The main task of the thermostat is not to supply coolant to the radiator until the engine reaches a certain operating temperature, usually, + 85 ° C (185 °F), thus, to transfer it to the so-called “big” coolant circuit. However, if the thermostat does not open in time and does not circulate the coolant through the system, then it will quickly heat up together with the engine and the coolant will simply boil since it will not have time to cool down.

3. Bad water pump

Since its task is to pump coolant, when it fails, its circulation stops, and the volume of liquid that is closer to the engine begins to heat up strongly and, as a result, boils.

4. Bad cooling fan

Its function is the forced cooling of the system elements with the same name. It is clear that if the cooling fan does not turn on, then the temperature will not drop and this may result in a boiling coolant. This situation is especially critical for the warm season.

5. The presence of air in the cooling system

The main reason for its appearance is the depressurization of the cooling system. As a result, several harmful factors appear at the same time, the pressure drops, which means that the boiling point of the coolant also decreases. Moreover, with prolonged presence of air in the system, the inhibitors that are part of the antifreeze deteriorate and do not perform their protective function. Finally, the coolant level drops

6. Low coolant level

If the tank is not filled to the required level, it will not cope with its task, therefore the temperature exceeds the critical level and the coolant boils.

low coolant level
low coolant level
source: jlwranglerforums.com

7. Bad temperature sensor

A bad or a failing temperature sensor does not send appropriate signals to the thermostat and/or cooling fan. They didn’t turn on and, as a result, the coolant is boiling.

8. Poor coolant quality

If a low-quality coolant is poured into the car, in most cases, a coolant that does not meet the requirements, that can mean that there is a high probability that the coolant will boil. In particular, we are talking about the counterfeit coolant that often boils at temperatures below + 100 ° C (212 °F).

9. Foaming coolant

This can happen for a variety of reasons. For example, poor quality coolant, mixing incompatible coolant types (example: blue with red), using an unsuitable coolant for your car, damage to the cylinder head gasket, which causes air to enter the cooling system, and as a result, its chemical reaction with coolant will lead to the formation of foam.

foaming coolant
source: motoringassist.com

10. Depressurization of the tank cover

The problem can be either the failure of the safety relief valve or the depressurization of the cover gasket. Moreover, this applies to both the coolant tank cap and the radiator cap. Because of this, the pressure in the cooling system becomes equal to atmospheric pressure, and consequently, the boiling point of the coolant decreases.

To restore the performance of the cooling system, and continue to prevent a situation where coolant boils, it is necessary to revise the above causes by the probability and frequency in which they fail.

  1. Expansion tank and its cap. This is especially true for cases when the coolant has boiled in the tank, and steam comes out from under it. Better to replace the entire valve cover.
  2. Thermostat. This unit must be checked if the radiator is cold when the engine is running, and the antifreeze is boiling. Also, it makes sense to check the thermostat after replacing the coolant, if it immediately boils.
  3. Cooling fan. It rarely fails, but it makes sense to check. As a rule, problems arise in dropped contacts or breakdown of the stator and/or rotor insulation windings.
  4. Temperature sensor. This sensor is quite reliable, but it sometimes fails on older cars. The sensor controls the operation of the fan on the radiator
  5. The water pump. Here it is similar to the previous point.
  6. The radiator. It should be carefully inspected for damage and possible coolant leaks. If it leaks, then it is necessary to remove the radiator and inspect it. In most cases, it is recommended to replace it with a new one. You can just clean it if it is very clogged. It is better to remove it for external cleaning. As for the internal cleaning, this takes place together with the entire cooling system (without removing).
  7. Check the level of coolant. It can leak out of the damaged system, and the remaining volume cannot withstand the heat load and will boil. If a poor quality liquid with a low boiling point is used, then it must be replaced completely. Otherwise, you can simply add coolant every time it drops.
  8. Check if the coolant is suitable for the current car. If there was a mixing of two brands of coolant, then make sure that they are compatible with each other.
  9. Check the function of the safety relief valve. You can check the operation of the valve on the cover using polyethylene.
  10. Check the quality of the coolant. Quality coolant is always an excellent choice. Do not be stingy with this liquid of special importance for the cooling system. If you are uncertain of the quality, simply change it.

    The consequences of adding bad or poor quality coolant are the boiling and destruction of the engine.

When does coolant boil?

Often, boiling occurs when the car is moving in a low gear, or when the engine is running at high speeds, for example, when driving for a long time on a long journey or in city traffic jams in the summer heat. The situation is worse if the air conditioning is turned on, since it puts additional stress on the cooling system, in particular, on the main radiator. 

Therefore, before going on a long drive, be sure to check the condition of the engine cooling system, including the level of antifreeze in it. 

TIP! It is not recommended to use antifreeze containing more than 60% by volume of ethylene glycol and less than 40% by volume of water.

Often the cause of the boiling coolant can be the formation of air in the cooling system. Symptoms of its formation are problems in the operation of the thermostat, coolant leakage, problems with the pump. Therefore, if at least one of the listed problems is on your car, then it is recommended to correct the situation, since ignoring it can do harm to the engine.

What are the consequences of a boiling coolant?

The consequences depend on how overheated the engine is. And this, in turn, depends on the brand of the car (engine power and body mass), the engine, as well as the time between the boiling of the engine and its stopping (the moment when it began to cool down). Let’s conditionally divide the possible consequences into three conditions: mild, moderate, and severe.

So, if the engine overheats slightly (up to 10 minutes), slight melting of the engine pistons is possible. However, they can change their geometry. In most cases, this situation is not critical, except if there were problems with the geometry before. If you notice the boiling of coolant in time and take appropriate measures, which will be discussed below, then it is enough to eliminate the causes. This represents the mild condition

An average-case of overheating occurs approximately 20 minutes after the coolant has boiled. This is the moderate condition from above. So, the following types of breakdowns are possible:

  • curved cylinder head housing (relevant when the engine temperature reaches +120 degrees C (248 + F) and above);
  • cracks may appear on the cylinder head (both very small and cracks visible to the human eye);
  • melting or burning out of the cylinder head gasket;
  • failure (usually complete destruction) of the inter-ring partitions on the motor pistons;
  • the oil seals will begin to let the oil through, and it can either leak out or mix with the boiled antifreeze.

The already listed consequences are enough to represent the scale of the tragedy that can happen to the car if the coolant boils. 

However, if the driver, for some reason, ignores the boil and continues to drive further, then the so-called severe condition sets in. In very rare cases, the motor can simply explode, completely burst, and fail, but this does not happen often. Typically, destruction occurs in the following sequence:

  1. Melting and combustion of engine pistons.
  2. During the above-mentioned process, the molten metal hits the cylinder walls, thereby hindering the piston movement. Ultimately, the piston will be destroyed.
  3. Often, after the pistons fail, the car simply stalls and stops. However, if this does not happen, then problems with engine oil will begin.
  4. Because the oil is also gaining a critical temperature, it loses its operational properties, which is why all the rubbing parts of the engine are put at risk.
  5. Typically, small parts melt, and in liquid form, they will stick to the crankshaft, which makes it difficult to rotate.
  6. After that, the valve sockets start to fly out. This leads to the fact that under the influence of at least one piston, the crankshaft simply breaks, or, in extreme cases, bends.
  7. A broken crankshaft can easily break through one of the cylinder block walls, and this is equivalent to a complete engine failure, and, such a motor is unlikely to be restored.

Obviously, the consequences of boiling coolant can be very sad for both the car and its owner. Accordingly, it is necessary to maintain the cooling system in order, regularly monitor the level of antifreeze, and, if necessary, to complete until it reaches the normal level. 

In the case when boiling still occurs, then you need to eliminate the problem as soon as possible.

What to do if the coolant is boiling?

The first thing to remember is not to panic. It is advisable to pay attention as early as possible to the fact that the cooling system is partially “damaged”. This can be done both with the help of instruments and visually by observing the steam coming out from under the hood. The sooner you take the appropriate action, the more likely you can get away with inexpensive repairs.

  1. Move the gear to neutral or idle to reduce engine RPM to idle.
  2. Continue driving, and do not drop speed suddenly. The counter-air will be blown to the engine as much as possible to cool it.
  3. While on the move, turn on the heat to the maximum at the highest possible temperature. Moreover, this must be done regardless of the season. This procedure is done to maximally remove heat from the radiator.
  4. You need to roll as long as possible, until a complete stop. If it happens in the summer, then it is advisable to find a place to stop somewhere in the shade, without direct sunlight. After that, the engine must be turned off. In this case, the ignition must be left on to let the heater work for another 5 – 10 minutes. Then turn off the ignition too.
  5. Open the hood to give maximum natural air access to the engine compartment. Without touching any engine parts with your hands, wait for a certain time. In summer about 40 – 50 minutes, in winter – about 20. Depends on the weather conditions and the time while the car was “boiling”.
  6. Call a tow truck or a vehicle that tows the car to a workshop or a good mechanic with appropriate diagnostic equipment.
  7. If there are no cars nearby, then after the mentioned time, making sure that there is no boiling and the liquid has “cooled”, carefully remove the cap of the coolant tank and add clean water. If you are not far away, you can use any non-carbonated drinks. Fill up to the mark.
  8. Start the car, turn on the heat to maximum and continue to drive at low speeds. As soon as the coolant temperature reaches + 90 ° C (194 F), you need to stop and wait again for 40 minutes (or 20 if it’s winter). If you’re not far away, you’re in luck. Otherwise, you need to look for an option with a tow truck.
  9. When you arrive in a workshop, tell the technicians about the problem, usually, they will easily find a breakdown (among those described above) and fix it.
  10. Also, be sure to ask them to change the coolant, since the liquid that is currently in the system has already lost its performance properties.

The above steps are fairly simple, and even an inexperienced driver can cope with it. The main thing is to notice in time the process of the boiling coolant itself. And it is advisable to always have a small supply of coolant in the trunk (preferably similar or compatible with the one used at the moment), like engine oil. It does not take up much space, but it can come in handy at a critical moment.

What not to do when the coolant is boiling

  1. Do not load the engine. Instead, you need to reduce the speed as much as possible to the idle value, usually around 1000 RPM.
  2. Do not suddenly stop and turn off the engine, thinking that the engine will stop boiling, on the contrary, everything will only get worse.
  3. Do not touch hot parts of the engine compartment!
  4. While steam is coming out from under the cover of the tank or other unit and while the antifreeze is boiling in the system, it is absolutely impossible to open the cover of the tank! This can be done only after the time indicated above.
  5. Do not pour cold water on the engine! You need to wait for the engine to cool down on its own.
  6. After the engine has cooled down and you have added coolant, you cannot drive after reaching a temperature of more than +90 degrees.

If you follow these simple steps, you will ensure your safety, and also you will minimize the costs.

main photo source: rx8club.com

Scroll to Top