Why is My Coolant Reservoir Boiling and How to Fix? Detail Guide

You know your car engine produces a huge amount of heat when you drive. But can you tell how it cools down all by itself and keeps running? Thanks to the cooling system of your car, you can drive miles without melting any parts inside the engine. But, what happens when your cooling system stops working?

The coolant reservoir of your car starts boiling and the car fails to perform optimally as a result. So, if you’re thinking, why is my coolant reservoir boiling and how to fix this issue, you’re in the right place.

The reason can be a faulty head gasket, damaged radiator cap, dysfunctional thermostat, worn out radiator fan, presence of air inside the reservoir, etc.

To help you find why your coolant reservoir might be boiling, here we will discuss all the possible reasons and tell you how to fix the problem easily. So without further ado, let’s jump right in.

Why is My Coolant Boiling and Smoking?

First things first, you need to understand the cooling system of your car to know the reasons behind coolant boiling in reservoir, car overheating, smoking, engine warning, and other issues.

As mentioned, the reservoir reserves and dispatches the coolant, and it’s directly connected to a radiator. The radiator drives the heat away from the coolant.

Other components of the cooling system include a water pump that circulates the coolant through the engine parts, a radiator fan for pulling the air through the radiators, and a thermostat for controlling the coolant temperature.

If there’s any fault in either one of these components, the temperature will rise inside the reservoir, reaching the coolant boiling point and causing the coolant’s bubbling, boiling, or smoking. The boiling point of a coolant depends on the amount of mixed water and pressure inside the reservoir. Typically, it ranges from 223°F to 267°F.

Now that the basics are off the table let’s talk about the possible causes of boiling and smoking coolant inside a reservoir.

Insufficient Coolant

This is probably the most common reason why your coolant might be boiling. Inside the reservoir, the coolant needs to maintain a standard level for optimum system performance. If the coolant level goes below standard, you should receive a low coolant warning.

In case you don’t receive the warning or continue to ignore it, the coolant will start boiling, and smoke might come out of the engine. You can easily solve this problem by using leak repair fluid or maintaining the standard coolant level.

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Air Pockets

When you refill the coolant, some air might just sip into your car’s cooling system and engine parts and create air pockets. Once the temperature rises, those pockets expand and interrupt the normal flow of coolant.

Moreover, the air pockets can easily reach the other components like the water pump and head gasket, causing the reservoir’s pressure to go lower than optimal. Due to the reduced pressure, the coolant easily reaches its boiling point and starts to form bubbles and smoke.

The air can also enter the cooling system through leaks and a damaged radiator cap.

Damaged Water Pump

The water pump circulates the coolant through the cooling tubes and various engine paths. If the water pump cracks or leaks, air enters the pathways and reaches various compartments.

Therefore, the pressure inside the cooling system decreases, and the coolant generates bubbles. Coolant reservoir bubbling after shutdown is the main indication of a faulty water pump. You’ll also notice coolant leaking from the reservoir if the pump is cracked.

Bad Radiator Cap

Besides sealing the entire cooling system and pressurizing it, a radiator cap also functions as a pressure relief valve. So, if the radiator cap is flimsy, it won’t be able to keep the coolant under the required pressure.

Hence, the pressure inside the reservoir will go down, causing the coolant to boil and create smoke. Besides, an incompatible radiator cap will let the air pass inside the system, eventually restricting the flow of coolant toward the engine components.

Dysfunctional Thermostat

Here’s how the thermostat works. It’s basically a valve that opens and closes at specific times to let the heated coolant flow from the engine to the radiator. The thermostat maintains a water temperature of 55°C, and it’s essential to keep the engine active.

When the thermostat fails to perform, it remains opened or closed longer than necessary, causing the engine and coolant to overheat. Besides boiling coolant and engine overheating, you might also hear weird noises if the thermostat malfunctions.

Blown Head Gasket

Another common cause of boiling coolant is a blown head gasket. If your car gets overheated regularly, there’s a high chance the head gasket will wear out or get badly damaged.

In that case, combustion gas from your car cylinders will enter the cooling system through the cylinder head. These escaping gases will release the overall pressure from the cylinders and create bubbles in the coolant.

Also, the gases will mix with the engine oil and coolant due to the blown head gasket resulting in loss of coolant and engine overheating.

Faulty Radiator Fan

The function of a radiator fan is simple but important. It assists the radiator in keeping down the temperature of the liquid coolant. The coolant then goes inside the engine components and keeps them cool while you’re driving.

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So, if the radiator fan is damaged or faulty, it won’t be able to cool down the liquid, and the hot liquid will go back to the engine, causing the temperature to rise high. This will overheat your car, cause the coolant to boil, and create unwanted steam in the cooling system.

How Do I Stop My Coolant from Boiling?

To find the solution, first, you have to know exactly where the flaw is. As you already know, which components of the cooling system and engine might create the boiling coolant problem, start by checking out all of them to find out the faulty part.

Below, we have listed some common ways of detecting faulty components and provided step-by-step methods to fix them. Let’s have a look.

Detecting and Fixing Air Pockets

Detection: If there are air pockets inside the cooling system, you’ll notice-

  • Car overheating while you’re driving
  • Loss of coolant more frequently
  • Heater malfunctioning
  • Water gushing or boiling sound from your dashboard

Fix: As you know, the air can enter the system in several ways. In most cases, the problem arrives after the refilling of the coolant. So, every time you flush and refill the reservoir, make sure there’s no remaining air.

To get rid of the air pockets, seal the reservoir with the radiator cap and run the engine for at least 15 minutes. This will help remove the air from the system. Once you’re done, place the cover back in place and start driving to check if there’s any sign of boiling coolant.

If the problem persists, maybe the problem is with the radiator cap, thermostat, or head gasket. In that case, proceed to the next fixes.

Detecting and Fixing Bad Radiator Cap and Faulty Radiator Fan

Detection: You can detect a faulty radiator cap and fan by the following signs-

  • Coolant leakage
  • Overheated engine
  • Collapsed radiator hose
  • Steam coming out through the vent
  • Overflowing coolant reservoir
  • Low coolant level

Fix: Unfortunately, there are no effective ways of fixing the radiator cap. You can use black pepper to seal a leaking radiator cap for some time. To permanently get rid of the related problems, you have to replace the faulty radiator cap with a new one.

While buying a radiator cap, make sure it’s compatible with your vehicle model. A radiator cap will cost you only $10 to $50. Since changing the radiator cap is a long, complex, and dangerous process, we recommend that you take your car to an expert to replace it.

On the other hand, replacing the radiator fan is a simpler process, and here’s how you can do it-

  • Step 1: First, wear your hand gloves and protective goggles. Stop your car and let the engine cool down. Then open your car hood and withdraw the air filter
  • Step 2: The radiator will have a few wires attached. Unplug all the electric connections and unscrew the radiator
  • Step 3: Remove the battery and the radiator hose. Now you’ll be able to access the radiator fan. Take off the damaged radiator fan and replace it with a new one. When you’re done, replace the removed parts, and that will be all.
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Detecting and Fixing Dysfunctional Thermostat

Detection: Here are the signs that indicate a dysfunctional thermostat-

  • Full expansion tank and rising temperature
  • Overheated engine
  • Erratic temperature changes
  • Leaking coolant
  • Heater fluctuations
  • Smoking engine

Fix: If you have a faulty thermostat, the best option is to replace it, and a new one will cost you only $30 to $50. Follow the steps below to replace the old thermostat-

  • Step 1: Wear your protective goggles and hand gloves. Then gather the necessary tools. You’ll need a ¼-inch ratchet extension and socket for this job.
  • Step 2: Drain the remaining coolant from the reservoir and locate the thermostat. It should be near the top portion of your car engine near the radiator hose. Use the extension and socket to remove the bolts that hold the thermostat in place and take off the dysfunctional thermostat.
  • Step 3: Remove the thermostat gasket as well and drop in the new thermostat. Make sure the spring side is faced down. After that, replace the bolts and radiator hose. Check if all the parts are placed nicely, and your job is done.

Detecting and Fixing Blown Head Gasket

Detection: The following signs will help you detect a blown or worn-out head gasket-

  • Engine overheating
  • White smoke from the tailpipe
  • Leaking oil and coolant
  • Low coolant level
  • Rough idle
  • Engine oil mixed with water and coolant

Fix: Repairing a blown head gasket will cost you almost the same as replacing it with a new one. So, it’s better if you replace it instead. The cost of a new head gasket is between $700 and $850.

As the replacement process is very complex, you have to take the vehicle to a professional. Overall, you might have to spend $1,200 – $2,000 depending on your car model.

Final Words

So, why is my coolant reservoir boiling, and how to fix it? There you have the answer. Make sure you address the boiling and smoking coolant issue as soon as possible.

Otherwise, it will cause permanent damage to your engine and other valuable car parts. Don’t hesitate to take help from an expert when necessary, as it will save you from more expensive fixes in the long run.

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Brian Polk

Brian is an automotive expert who has 12 years of experience in the industry. He has worked on a variety of cars, from high-end luxury vehicles to budget-friendly options, and has a wealth of knowledge on the subject. He is passionate about automobile and enjoys sharing his knowledge with others, whether they are looking to purchase a new motorized vehicle or simply learn more about the inner workings of these machines. In his free time, Brian enjoys working in his automobile workshop and spending time at the track.

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