ATV Coolant Boiling: Why It Happens & What to Do

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You are out on the trail, and out of the blue, steam begins pouring out of the ATV. This post covers why ATV coolant boils and what to do when it happens.

How to Know if the Coolant Is Boiling

Boiling coolant is a surefire sign that the ATV engine is running too hot. Besides the other signs and symptoms of an overheating ATV, here is how to tell if the coolant is boiling. 

You can hear it. When you shut off the ATV engine or idling, you can hear the coolant boiling in the pressure tank and radiator. 

You can see the coolant boiling in the pressure tank. The pressure tank is made of opaque plastic, and if the coolant is boiling, you can see it through the plastic. 

Coolant leaks through the coolant reservoir overflow tube. The cooling system is designed to let out some coolant if the pressure inside the cooling system or the coolant level becomes too high. Look for an overflow tube on the top of the radiator or the coolant reservoir. 

Caution: Never open the radiator or pressure tank cap when the coolant is hot or boiling. If the lid is opened, the hot coolant is under high pressure and will shoot out. Always wait until the radiator is cool before opening the radiator or coolant reservoir.

Common Causes When ATV Coolant Is Boiling

Debris on the Radiator Screen

Many ATVs have a protective screen in front of the radiator to protect the fragile aluminum fins and keep dirt and debris away. If the screen is caked with mud, it will not allow cool air to flow, and the radiator cannot do its job. 

The solution is relatively simple: Remove and clean the screen before putting it back in place. 

Plugged Radiator 

If your ATV has no protective screen or you’ve been riding in extremely muddy conditions, the radiator may have become dirty, restricting the airflow. 

Use a garden hose to flush any mud or debris from the radiator fins. Do not use a pressure washer, as the high-pressure water may deform the radiator fins, significantly reducing its cooling efficiency. 

Bad or Stuck Thermostat

The thermostat is a component in the cooling system that opens and closes to control coolant flow depending on water temperature. 

The thermostat is supposed to open when the water temperature reaches a specific threshold, but if it has become damaged or is stuck from corrosion, it may not open.

When this happens, the water cannot circulate through the system and quickly reaches the boiling point. 

Faulty Radiator Fan

Most ATVs are equipped with an electrical cooling fan that activates when the water in the system reaches a specific temperature. If the fan motor or control switch is faulty, the fan may not turn on, causing the coolant to boil when you ride in hot conditions. 

The fan makes an audible noise when it turns on, which should be easy to hear. Do not attempt to touch the fan to see if it is on; it has enough power to cause severe finger injury. 

Faulty Water Pump

The water pump’s function is to circulate coolant throughout the cooling system. If the pump or impeller fails, the circulation stops and does not reach the radiator to cool down. 

Faulty Radiator Cap or Leak

The cooling system is designed to operate under pressure to raise the coolant’s boiling point. High-quality coolant can reach 266ºF (130ºC) before it begins to boil.

If a faulty radiator cap or a leak drops the pressure in the system, the coolant will boil much sooner. 

Other possible causes include underperforming coolant, trapped air in the cooling system, or a damaged radiator.

Coolant Reservoir Boiling but Engine Not Overheating?

When the coolant reservoir is bubbling, but no signs indicate the ATV is overheating, the bubbles are likely not caused by boiling water but by an air leak in the cooling system.

Common causes include a leaking radiator cap, broken water pump seal, or blown head gasket. 

The cooling system is pressurized, and when air enters the system, it naturally rises to the highest point, and the pressure causes a boiling effect. 

What to Do if the Coolant Is Boiling in Your ATV

The most important thing to do when the coolant boils is to stop riding and allow the ATV to cool down. Continued operation of an ATV that is overheating can cause severe damage to the engine. 

If the radiator is caked with mud, pour water over it to cool it down and remove some dirt. 

Allow the ATV to cool down, and begin your ride home as slowly as possible. Stop and allow the ATV to cool if it starts to overheat again. 

Modern ATVs usually have a limp mode that significantly reduces engine performance to protect the engine if it senses it is overheating. 

If the coolant level is low, avoid adding water from a creek or pond into the cooling system, as it may cause contamination and corrosion.

If you have no other option than adding water to return home, perform a proper coolant flush as soon as possible to remove any contaminants before they cause too much damage.

Haavard Krislok
Haavard Krislok
Haavard Krislok is an ATV and off-road enthusiast with a rich background spanning two decades in owning, maintaining, repairing, and utilizing ATVs for farming, logging, and hunting. Outside his professional life as an engineer and project manager, he cherishes recreational trail riding and is the creative force behind, serving as its owner, editor, and content creator.

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