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9 Reasons Coolant is Boiling After Car is Turned Off

Coolant Boiling After Car Turned Off

Boiling coolant after you’ve turned your car off could be the result of many things.

An automobile’s cooling system is an important part of the engine. It’s made up of a network of hoses designed to deliver coolant throughout the engine.

Coolant is an integral part of the cooling system that also includes the radiator, thermostat, water pump and hoses. But if air enters this system. It can block coolant from reaching any one of these mechanical components causing coolant to boil, even after you’ve shut the motor off.

We’re going to look at various reasons why coolant is boiling after you turn off your car. But the most important thing to know is if you don’t take action now, this can result in bigger problems down the road.

1. Low Coolant

Coolant Boiling After Car Turned Off Could Be the Result of a Low Coolant

The cooling system is a closed system. Specifically, this means that it’s pressurized. And if anything enters the system that’s not supposed to be there, it can wreak havoc on your car.

Engine coolant is a mixture of antifreeze and water formulated to keep an engine from overheating in hot weather and freezing in cold weather. Along with regulating engine temperatures, it also helps prevent corrosion and rust in other parts of the cooling system.

The boiling point for coolant is 300-to-400-degrees Fahrenheit depending on the engine, type of coolant and coolant mixture.

But if there’s a leak in the cooling system, pressure drops. This lowers the boiling point for coolant which is one of the reasons why it can bubble – this can look like boiling coolant.

2. Faulty Radiator Cap

Coolant Boiling After Car Turned Off Could Be the Result of a Faulty Radiator Cap

Boiling coolant can be such an alarming experience that it’s easy to miss the obvious.

Due to its simplicity, the radiator cap is an often-overlooked part of vehicle maintenance. But a radiator cap does a lot more than making sure coolant doesn’t spill all over the engine.

It also operates as a coolant release valve while maintaining the pressure of the cooling system. When coolant reaches a certain high temperature (before the boiling point) the release valve opens.

But if the radiator cap is malfunctioning, not only will it not be able to maintain pressure.

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It will also invite air bubbles into the cooling system which will obstruct coolant from reaching critical parts of the engine, causing the coolant to bubble which can also look like boiling coolant.

3. Thermostat Stuck Closed

Coolant Boiling After Car Turned Off Could Be the Result of a Thermostat Stuck Closed

Above is an image of a bad thermostat on the right, and a replacement thermostat on the left.

When the thermostat in your car isn’t working properly. The temperature gauge will shoot up dramatically. It can show that the engine is overheating almost immediately after turning on the car.

When the thermostat was bad in my sister’s 1994 Saturn SC2, the temperature gauge would shoot to overheating moments after you turn on the engine. It was alarming at the time because I didn’t know that it takes a while for an engine to warm up, much less get to the point of overheating.

A malfunctioning thermostat usually often means that the valve is stuck in the closed position. This prevents coolant from flowing into the areas of the engine that need it. Since the coolant is restricted from flowing to the rest of the engine, it boils in the reservoir.

A working thermostat will open and close at the optimal time to regulate the temperature of an engine.

4. Clogged or Damaged Radiator

Coolant Boiling After Car Turned Off Could Be the Result of a Clogged or Damaged Radiator

Damage to a radiator could be the reason why it’s clogged. The condition of the radiator is another wear and tear item worth checking from time to time.

A radiator’s primary function is to monitor and regulate an engine’s temperature to keep it from overheating.

Coolant travels through hoses from the radiator. It’s then distributed throughout the engine to absorb excess heat, passing through the radiator’s fins where it releases the engine’s heat into the atmosphere as it gets recirculated back to the radiator.

But debris can find its way into the radiator causing it to get clogged. And since this keeps it from distributing coolant to the rest of the cooling system, the coolant might start to boil.

5. Failing Radiator Fan

Coolant Boiling After Car Turned Off Could Be the Result of a Failing Radiator Fan

Attached to the radiator is the radiator fan. It’s designed to kick in when your engine reaches temperatures of around 200 degrees Fahrenheit.

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In some modern cars the radiator fan will keep working to cool the engine down even after you’ve turned off the motor. Its primary job is to keep the coolant cool which in turn keeps the engine cool.

But a:

  • blown fuse,
  • damaged wire,
  • failed fan clutch,
  • or corrosion

can keep the fan from doing its job. And since the radiator fan is tied into the rest of the cooling system, it can cause the coolant to boil.

6. Bad Temperature Sensor

Coolant Boiling After Car Turned Off Could Be the Result of a Bad Temperature Sensor

There are many temperature sensors located throughout a car. The one for the engine can be either referred to as a CTS (coolant temperature sensor), or ECTS (engine coolant temperature sensor).

You can usually find it somewhere near the thermostat. The temperature sensor keeps tabs on engine temperature by measuring the temperature radiating from either the thermostat or coolant itself.

This information is sent to your car’s on-board computer (the ECU) to adjust engine function and keep engine temperature at an operational level.

If it’s not working properly, not only can the problem present itself as coolant boiling after you turn off your car.

But symptoms also include:

  • poor performance,
  • bad fuel economy,
  • black smoke coming from the exhaust pipe,
  • or a faulty water pump.

7. Bad Water Pump

Coolant Boiling After Car Turned Off Could Be the Result of a Bad Water Pump

On average, an engine’s water pump is designed to last 100,000 miles.

The water pump plays a vital role in engine temperature management. As hoses circulate coolant throughout the exterior of an engine, a water pump is responsible for injecting coolant into the engine to prevent overheating.

Usually when this component fails, it will be accompanied by a check engine light along with high readings from your temperature gauge. (My wife and I have recently experienced this issue in our Ford Fiesta. Turns out it wasn’t the water pump at all. But rather the ancillary components attached to the water pump that have become old and warn.)

Modern water pumps are more robust. But issues can be traced back to a worn out bearing or a leak in the pump itself.

Another obvious sign of a bad water pump besides boiling coolant is puddles of antifreeze on your parking space. (We didn’t have that, or at least I never saw it.)

8. Blown Head Gasket

Coolant Boiling After Car Turned Off Could Be the Result of a Blown Head Gasket

A head gasket has the very important job of sealing the combustion chamber to help the engine maintain the amount of compression it needs to make power.

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It also keeps fluids like coolant and oil from entering the combustion chamber.

Without a head gasket, your engine could catch fire. A damaged head gasket means that oil and coolant can get into the combustion chamber which is where fuel and air mix to make power.

I owned a Dodge Stealth with a blown head gasket. When this happens the engine’s temperature will surge which can result in boiling coolant.

Besides boiling coolant, you can also tell if the head gasket needs replacement when you see white smoke coming from the tail pipe like my car did. Or if you see a milky white color in the oil.

9. Turbo Engine

Coolant Boiling After Car Turned Off Could Be the Result of a Turbo Engine

Today, turbocharged engines are designed to have the same life expectancy as naturally aspirated engines.

However, since the point of a turbo is to eek out extra performance. This inevitably adds more stress to an engine. This means turbocharged engines work harder at higher pressures, and higher temperatures than a naturally aspirated engine.

Boiling coolant after you turn off the engine is a sign that the engine isn’t receiving proper cooling.

This can be remedied by installing a larger air intake which will increase the amount of air going to the turbo. Fixing the problem can even be as simple as replacing a dirty air filter.

A big reason why it’s so important to take action if you notice coolant boiling after you turn off your car is because an engine that isn’t properly cooled means oil won’t be able to keep parts lubricated.

An engine without enough oil causes friction resulting in catastrophic damage.