Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

When a car overheats, the first thing we blame is a low coolant level. However, did you know that a car can overheat even with full coolant? If your car is overheating with full coolant, read on to find out more about what can cause that.

If your car is overheating with full coolant there is air trapped in the cooling system that prevents the coolant from circulating properly. This air pocket can even confuse the thermostat and cause it not to open and allow hot coolant to escape from the engine, leading to overheating.

Key Takeaway

  • Air trapped in the cooling system can restrict the coolant flow and cause the engine to overheat.
  • If the thermostat is stuck closed, it will not allow the hot coolant to escape from the engine which will increase the temperature and the engine will overheat.
  • A seized water pump can cause the engine to overheat because the coolant needs to circulate around the engine to cool it down. Without the water pump, that is impossible.
  • If the belt that drives the belt is loose or broken, the water pump will have no power and the engine will overheat due to the lack of coolant circulation.
  • A blown head gasket can disrupt the regular flow of coolant around the engine and cause the engine to overheat.
  • A clogged cooling system can restrict the coolant flow and the engine will overheat.

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

Cars overheat if there is no coolant in the radiator. However, if your car is overheating with full coolant, here are six common causes and fixes that will help you.

1. Air trapped in the cooling system that is limiting the coolant flow

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

When you are adding coolant to the cooling system, the radiator may fill up to 100%. However, the engine may only fill up to around 80%, leaving about 20% air. Because of the air pocket, the coolant may not be able to circulate properly. The thermostat may not even open up allowing the hot coolant from the engine to enter the radiator and cooler coolant from the radiator to enter the engine to cool it down. This can lead to engine overheating even with full coolant.

Some vehicles are pretty straightforward and will bleed or burp by themselves. All you have to do is add the coolant to the radiator. Let the engine run for about 10 minutes. When the thermostat opens up, that’s gonna burp or bleed the air out of the cooling system. Then you shut off the vehicle and let it cool off and add the missing coolant.

But, those are the easy ones. Some vehicles take a little more work. In some vehicles, you may find that there is a radiator cap attached to the radiator. On those, you can add the coolant pretty easily. However, there are some vehicles that have an expansion tank. When trying to bleed a cooling system that has an expansion tank, you either need to bleed the air with a bleeder screw (if available on your vehicle), or with an air lift system.

2. The thermostat is frozen shut

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

A radiator thermostat helps the driver maintain the temperature of their car’s engine by regulating the flow of coolant from the system. The thermostat works by opening and closing at preset temperatures to regulate how much hot coolant runs through the radiator, letting it flow to or away from the engine depending on how hot it is.

When the engine is cold, the thermostat is closed and all of the coolant flows around into a bypass so that it can preheat before going through the radiator. As temperatures inside the engine increase, either from driving or from being out in the sun, gas is driven out of wax capsules within the thermostat housing, causing it to expand which in turn pushes a wax piston against a valve that separates outgoing and incoming coolants. This valve opens when the hot coolant needs to be allowed back into circulation in order to cool down your engine.

However, when the thermostat is frozen shut, it will not be able to open and allow hot coolant to escape the engine and replace it with cooler coolant. This can lead to engine overheating, even when the coolant is full.

To check for a faulty thermostat, start your vehicle and let it run. Open the hood and wait for the vehicle to reach the operating temperature which is about 210 degrees Fahrenheit. When the optimal engine operating temperature is reached, the thermostat should open. You can hear this because the coolant will start fowling and the radiator fan will kick in.

However, if nothing happens, there is a good chance that the thermostat is bad and isn’t engaging when it should.

3. Seized water pump

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

A water pump is an essential component in a car’s engine, playing an integral role in the cooling system. It works by taking coolant from the radiator and sending it to the engine block. This helps to regulate the temperature of the engine, ensuring that it won’t overheat.

The water pump needs to be functioning properly for the car to run at optimum levels and provide the driver with the desired performance. In order for it to remain effective, it must work in concert with other components such as thermostats, hoses, and radiators.

When a water pump seizes up, it can cause the engine to overheat regardless of how much coolant is in the system. This is because, without an engaged water pump, the coolant will not continuously flow through the engine. The system relies on this to prevent dangers such as overheating.

Checking whether a water pump is functioning correctly in a car is an important preventative maintenance step. A key indicator of a faulty water pump is a decrease in coolant levels or steam coming from under the hood. Both of these signs require immediate attention.

Open the hood and locate the water pump. Look for any obvious signs of damage to both the water pump and the belt that drives it. Start the engine and just observe the water pump. Check if the water pump belt starts spinning. Look for any leaks or abnormal sounds coming from the area where the water pump is located.

4. Broken belt that drives the water pump

Car Is Overheating With Full Coolant [6 Common Causes and Fixes]

The way that water pump is typically powered is with a belt, also known as a drive belt. It takes power from the crankshaft to spin the pump, allowing coolant to flow. This belt can become worn over time, requiring replacement in order to ensure the ongoing functionality of the vehicle.

When the belt that drives the water pump breaks, it will no longer power the water pump. From there, the water pump will not be able to draw hot coolant from the engine and send cold coolant from the radiator. This will result in engine overheating, even with full coolant in the system.

To check the belt, locate it by looking under the hood and identifying which belt is connected to the water pump. Once you have located the belt, look at its condition to determine if it needs replacing.

If there are worn grooves in the material surface, cracks, or fraying on the edges then it is time for a replacement. While checking, make sure that all associated components such as hoses and seals are also in good condition so that no additional problems crop up in the near future.

5. Blown head gasket

The head gasket of a car is one of the most essential components to ensure the engine runs efficiently. Not only does it form a seal between the head and the cylinder block, but it also provides combustion pressure control, stops coolant and oil from mixing, and helps distribute heat evenly throughout the engine.

Without a properly sealed head gasket, your vehicle would eventually suffer mechanical failure caused by combustion leakage and decreased cooling efficiency.

A blown head gasket can disrupt the flow of coolant through the tiny passages around the engine. As a result, the heat within the engine will build up and cause the engine to overheat.

Checking for a blown head gasket can be an intimidating process. But, with a few simple steps, you can determine if your car has this costly issue. Begin by inspecting the color of your vehicle’s engine antifreeze. If it looks muddy, cloudy, or rusty, that could signal a leak from the head gasket.

Additionally, listen carefully for hissing noises near the front of the engine, which could indicate exhaust leaking into the cooling system. Finally, check oil levels and look for a milky residue deposited on the dipstick or filler cap. This is a sign of water leaking into the crankcase.

6. Clogged cooling system

Dirt, grime, and other debris can cause a car’s cooling system to become clogged, leading to inefficient engine performance. This happens when the dust accumulates in radiators and deflectors. Also, when water pumps are not serviced or replaced on a regular basis. If a poor seal exists between the engine coolant reservoir tank and the radiator cap, dirt and debris may also be drawn into the coolant system.

With the gradual increase of clogging each component of the cooling system becomes less efficient and hence fails to keep optimum temperature which leads to overheating of engines. To avoid this issue, it is advisable for car owners to get their vehicles serviced more regularly with fresh fluids that help keep debris from sticking to their parts.

Igor Iwanowski

I am a certified Automotive Service Excellence (ASE) mechanic since 2018. I specialize in Brake systems, dashboard warning lights, EGRs, general engine problems, EVAP and Emissions issues.

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