Coolant Boiling After Car Turned Off: 6 Reasons Why It Happens


Did you find the coolant boiling after car turned off and was parked in your car garage for some time? Worry not, we will help you understand why this is happening.

A coolant is a special fluid that prevents engine components like the cylinder head from overheating. It comprises ethylene glycol, water, additives, and green, pink or blue.

One of the common problems most car owners face is that coolant boiling after the car turns off. There can be various potential reasons, like low coolant in the reservoir or a damaged electric water pump. The radiator can also be choked with time and can create such issues.


Coolant Boiling After Car Turned Off


5 Reasons Why Your Coolant Might Be Boiling

The coolant plays an essential role in keeping all the engine components cool. However, other components also play a role in cooling the car. They are the water pump, radiator, and water pump. Problems with any of these components can also make the vehicle engine overheat. Let us discuss the reasons for the coolant boiling after the car is turned off in detail.

#1. Low Coolant In The Reservoir

Low coolant in the reservoir can be the reason for the coolant boiling after you switch off your car. Low coolant can even decrease fuel efficiency and can reduce engine performance. You need to check the coolant level as soon as you notice the problem. If you find it low, then pour coolant into the reservoir.

Experts recommend that you need to flush off the old coolant after riding 30,000 or approximately after two years.

#2. Bad Radiator

The radiator is essential in cooling the engine components and is located under the car hood. The car engine produces a lot of heat while burning an air-fuel mixture. The heat needs to be vented to keep the engine components cool.

The thermostat is located on the engine front and detects the excess heat in the engine components. The coolant is then liberated from the radiator, flows through various parts of the engine, and extracts heat.

Coolant flows back to the radiator, exchanges heat with the outside air, and cools down. When the thermostat again detects excess heat in the engine, then the coolant is released from the radiator. This process continues repeatedly and helps to keep the engine components to become overheating.

But if you don’t flush the radiator within a gap of at least 5-6 years, dust, dirt, grease, and other foreign materials get trapped in the radiator pipes. Consequently, it reduces the efficiency of the radiator. The coolant cannot be adequately cooled down, resulting in the engine overheating.

Some signs of the blocked radiator are boiling coolant, an overheated engine, and smoke from the tail pipe. So, clean the radiator to remove debris once you observe such symptoms.

You can do the cleaning easily at home.

Remove the radiator cap and pour distilled water into it. Close the lid and start the car. Allow the car to run for almost 15-20 minutes. It will help in flushing all the dirt and debris from the radiator.


Coolant Boiling After Car Turned Off


#3. Faulty Radiator Cap

A damaged radiator cap is a primary reason for the coolant to boil off. Most car owners overlook the importance of the radiator cap. It no doubt looks so small but plays a vital role in cooling the engine components.

The radiator cap is a mechanical device that helps maintain the coolant pressure inside the pipes so they will not boil. After extracting heat from the engine components, the coolant starts to boil at 212 degrees Fahrenheit and changes into a vapor state. But the vapor coolant will b ineffective in cooling the engine components, and finally, your engine will get overheated.

The radiator cap helps maintain pressure inside the radiator and thus increases the boiling point of the coolant. The cap even prevents the outside air from entering the cooling system.

So, in short, the radiator cap acts like a pressure relief valve.

Most of the cars are equipped with radiator caps of 15psi. The valve will open when the pressure of the flow of coolant inside the radiator becomes more than 15 psi. It will release the extra pressurized coolant to the expansion coolant tank.

The coolant contracts when the engine cools down.The pressure in the coolant drops, and the engine radiator caps open the second valve, i.e., the vacuum valve. Now the coolant from the expansion tank flow inside the radiator.

But when the radiator cap malfunctions, it fails to check the coolant pressure inside the radiator. Hence the coolant may boil even after shutting the engine. A few other signs indicate malfunctioning of the radiator cap, like leakage of coolant or collapsing radiator hose. Check the radiator cap, and if you find it’s damaged, replace it with a new one to fix the problem.

#4. Faulty Water Pump

One more component of a car cooling system might be acting up in the water pump. The primary purpose of the water pump is to circulate the coolant from the radiator to various engine components.

The pump is attached to the engine’s crankshaft by a belt pulley arrangement. The impeller blades of the water pump move the coolant to the various components of the engine, thus preventing overheating.

The water pump has several parts which can damage with time. For instance, the plastic impellers can get worn out, or the bearings can get damaged.

In such cases, the water pump fails to work and creates problems for the vehicle. The coolant will start to boil both in the running and shutting conditions. The engine components will also get overheated, lessening the life of various vital components.


Coolant Boiling After Car Turned Off


#5. Damaged Cooling Fan

The cooling fan continues to run even after turning off the engine. It runs till the coolant cools down completely. It’s all because of ECU(Electronic control unit in the vehicle).

The ECU informs the fan that there is hot coolant in the chamber and needs to run until the coolant cools down.

But the cooling fan can damage with time. For instance, the blades can break or any derbies stuck inside the fan, and finally, the fain fails to work. Consequently, the coolant will be at a high temperature even when you turn off your engine.

Inspect the cooling fan when you observe such a problem. If you find it badly damaged, replace the cooling fan with a new one.

#6. Faulty Thermostat

This is not a reason for the coolant to boil but rather a case of wrong reading. It is possible that you have a faulty temperature gauge in the car and therefore are getting the wrong operating temperature reading. In such cases, just inspect the temp gauge by starting the car and see if the temperature is rising or not. If its does not happen, then it is possible that the coolant temperature sensor itself is busted.

Frequently Asked Questions

#1. Why is my coolant bubbling after the shutdown

The cooling system in most cars is pressurized and depends on hoses for pumping coolant to different engine components. But sometimes, the air gets inside the closed system and forms air pockets. This often happens when there is a blown head gasket in the engine.

They can create blockages which finally cause air bubbles to rise up in the coolant and then cause engine overheating.


Coolant Boiling After Car Turned Off


#2. Why would my coolant be boiling?

If your coolant is boiling, then there can be several reasons for it like if there is low coolant inside the reservoir, then you will face problems. Secondly, if the radiator tubes are blocked by grease, dust, and dirt or the radiator cap is not working properly, you will face such a problem.

#3. How do I stop my coolant from boiling?

You can stop the coolant from boiling in numerous ways. For instance, check the coolant reservoir from time to time. If it’s low, then pour some coolant into it. Secondly, flush the radiator within a few years to remove all the dust and debris. Inspect the cooling fan blades also from time to time. If it’s damaged, then replace it with a new one.

#4. Is engine coolant supposed to boil?

Yes, the engine coolant can boil. However, its boiling point depends on several factors. Some of these factors could be things like what pressure is present in the cooling assembly and the quality of the coolant itself. For instance, if we talk about Prestone coolant, it boils at 129 degrees Celsius.


Coolant Boiling After Car Turned Off


Wrap Up

Coolant is a special fluid that helps in keeping the engine cool. It has a high boiling point and is thus able to withstand engine heat without changing its phase. But if it boils after turning off the engine, it can be because of a blocked radiator, engine block, malfunctioning of the radiator cap, damaged fan blade, or problems with a bad water pump.

Try to inspect these components and, if required, replace them. Simultaneously check the coolant reservoir. If it’s low, then add some more coolant to it. Thank you for reading this article.

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Sean Mendez

Hi, I am Sean, a self-confessed petrolhead. I live in Boise, Idaho with a busy family of four and our energetic Labrador retriever. Thank you for visiting my website. You can find my email on the contact page.

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