Here’s why coolant boils after the car is turned off

Often I get asked this question. If you have a car with coolant that behaves this way, you might be grappling with this question. Fortunately, I have an answer.

A boiling coolant after turning off the car is a common problem these days.

Your coolant could be boiling even after you shut off the engine because of many reasons including a low coolant level, clogged radiator, bad radiator cap, and defective water pump.

Don’t forget that an overheated engine will take about 30 minutes to cool down. That means if you just shut off your engine 10 minutes ago or so, the coolant may continue boiling as the temperature drops gradually.

What Are the Causes of a Boiling Coolant After The Engine Is Off?

The short answer is anything that will cause coolant to boil when the engine is running. Remember that the engine will take a while to cool down when shut off.

As stated earlier, these are;

1. A Low Coolant Level

Boiling coolant can be a sign that your coolant level is below the recommended level. Boiling can be caused by decreased thermal resistance and the overall performance of the coolant.

Solution: Top your coolant level back up. And as a side note, experts recommend flushing and replacing the old coolant every 30,000 miles, or 2 years.

2. A Clogged Radiator

Just like other components of your car, your radiator requires routine maintenance so that it delivers optimal performance consistently. Speaking of which, it’s a good practice to clean and flush your radiator once every 100,000 miles or five years.

Otherwise, dirt, oil grime, and debris will pile, eventually clogging it. When that happens, water circulation within the radiator will be blocked, causing the engine to overheat and the coolant to boil. Matter of fact, boiling coolant is one of the surefire signs of a clogged radiator.

Solution: Unclog your radiator by cleaning away the debris deposited in it. N/B: To avoid damaging the delicate components, avoid using tap water. Instead, use deionized water without electrically charged atoms or molecules that can cause damage to your radiator.

3. A Faulty Radiator Cap

Though often overlooked, the radiator cap is one of the chief causes of boiling coolant. Functioning as a pressure valve set to open at a given pressure point(15 psi), the valve will automatically open when the pressure of the coolant flowing within the radiator exceeds this value.

This action will slightly depressurize the coolant, lowering its temperature(remember that pressure leads to heat) and thus preventing it from boiling.

Unfortunately, the radiator cap is not immune to failure. When it fails, it won’t be able to keep the coolant’s pressure in check. Consequently, the fluid mixture will end up in a boiling state.

Solution: Check and replace your radiator cap if it’s faulty. Besides boiling coolant even when the engine is shut off, other signs of a faulty radiator cap include leaking coolant, an overflowing reservoir, and a radiator hose that keeps on collapsing.

4. A Bad Water Pump

The water pump is another crucial component of your vehicle’s cooling system. Its role is to pump your coolant from the radiator to the engine block so that it doesn’t overheat.

Unfortunately, a water pump is susceptible to failure due to natural wear and tear among other factors. And if it fails, that can mean lots of problems for your car, including having your coolant boiling both when the engine is running and when it’s shut off.

Solution: Check and replace your water pump if it’s faulty. Apart from boiling coolant even when the engine is not running, other signs to look for include leaking coolant, overheating engine, as well as high-pitched, harmonic whining noises from the pump.

5. A Bad Cooling Fan

Did you know that the cooling fan continues running when the engine is shut off until when the coolant has cooled off? It’s all thanks to the ECU(the engine’s brain), which can tell that the coolant is still hot when you shut off the engine.

The Engine Control Unit, therefore, makes sure the fun continues to run until the engine cools down and the coolant is disengaged.

If the cooling fan is not functioning properly, it may not sufficiently lower the high temperatures of the coolant that will be registered even when the engine is shut off. Hence you’ll find that your coolant is still boiling even after shutting off the engine.

How Do I Stop My Coolant From Boiling?

By fixing the cause of that, which can be a low coolant level, clogged radiator, faulty radiator cap, bad water pump, or bad cooling fan.

Mostly, these components are better replaced than repaired. By replacing, you can be sure that your vehicle will cover many miles before again having issues with the component. The same can’t be said for the repair alternative.

How Much Does It Cost To Fix This Coolant Boiling Problem?

The answer depends on the cause of the problem and whether or not you are going to need the services of a mechanic. For example, if it’s a low coolant level, you’ll have to part with $20 for a new coolant.

As for a clogged radiator, be sure to part with anything between $100 and $200 to have it unclogged by a professional. For a faulty radiator cap, replacing that will set you back $10-$50 minus labor.

As for a bad water pump, you’ll need to part with $200-$400 to replace that with a new one. Lastly, if it’s a bad cooling fan that needs replacing, be informed that new cooling fans go for $550-$650.


Your coolant is designed to cool your car’s engine without boiling. After all, coolants have a high boiling point. This is what enables them to withstand your engine’s temperature without changing state.

However, if it’s boiling even after shutting off the engine, that can only mean one thing—your engine is overheating. This problem can be caused by many issues including a low coolant level, clogged radiator, faulty radiator cap, bad water pump, and failing cooling fan.

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