Your car's engine is filled with thousands of mechanical and electrical components that work in harmony to shuttle you around. When one system fails, though, it can leave you stranded on the side of the road. 

Few system failures are as potentially catastrophic as a cooling system failure. When the cooling system fails, the engine can overheat, which can cause severe internal engine damage, crack the engine block, damage the cylinder heads, and more. 

To diagnose why your car overheats, you must know where to look first. Below, we'll cover the common causes of a car overheating, some of the symptoms to look for, and what to do.

What a Cooling System is and How it Works

Before getting into common causes of a car overheating, it's best to start with a general understanding of the cooling system and how it works. 

What Is a Vehicle Cooling System

A vehicle cooling system regulates the temperature of the engine by using a liquid to keep the engine temperature in a safe operating range and keeps it from overheating under high stress. It also provides the heat the heating system needs to warm up the interior on cold days. 

The cooling system comprises a range of components, including: 

  • Coolant
  • Coolant reservoir
  • Radiator
  • Radiator fan
  • Thermostat
  • Temperature sensor
  • Radiator hose
  • Heater hose
  • Thermostat
  • Water pump
  • Heater core

How a Cooling System Works

When an engine is sitting cold, there is coolant -- a mixture of water and antifreeze -- in the radiator, coolant reservoir, and engine. Once you start the engine, the internal combustion process creates heat that warms up the engine. The thermostat opens as the engine reaches its defined operating temperature, allowing coolant to flow through the radiator and heater hoses and into the radiator via the upper radiator hose. 

In the radiator, the coolant flows through small passages or veins and is cooled by the air the radiator fan draws over the passages. Once the coolant reaches the lower radiator hose, it flows back into the engine to absorb more heat and maintain the engine's temperature. 

This heated coolant then flows through the heater hoses and back to the heater core to warm up the cabin (if needed). The coolant then flows back into the engine to absorb more heat, then through the thermostat and upper radiator hose to return to the radiator for cooling again. 

During all this, the radiator fan is alternating off and on to regulate the coolant temperature based on readings from the temperature sensor. The only exception is if your car has a clutched fan. In this case, the fan clutch engages and disengages with the under-hood ambient temperature, which regulates the intensity that air is drawn across the radiator passages. 

If one component in the cooling system fails, it can cause a vehicle to overheat.

Common Causes of a Car Overheating

There are many reasons for an engine overheating, ranging from low coolant to a failed water pump. Let's explore the most common causes of this frustrating mechanical failure. 

1. Stuck Thermostat

The engine's thermostat should open at a specific temperature to allow the coolant to flow into the radiator for cooling. This temperature is typically anywhere from 82 to 93 degrees Celsius.

Over time and from not changing the coolant regularly, gunk buildup, corrosion, and other debris from the coolant system can cover the thermostat, preventing it from properly sensing the coolant temperature. In some cases, the thermostat fails mechanically and simply won't open. In either case, this leaves the coolant stuck in the engine, resulting in the engine overheating relatively quickly. 

Tell-tale signs of a bad thermostat are bloated radiator and heater hoses. You'll also notice the coolant reservoir constantly remains at the "Full Cold" mark and never reaches the "Full Hot" line. 

2. Failed Water Pump

Over time, the water pump bearings, impeller, and other components can fail, preventing it from pushing the coolant along. Without this pump, the flow of coolant to the radiator stops, causing overheating.  

Tell-tale signs of a failed water pump include metallic debris in the coolant, a grinding noise from the front of the engine, and leaking coolant from the front of the engine.

3. Malfunctioning Radiator Fan

The radiator fan has the important job of drawing air over the coolant as it passes through the radiator, reducing its temperature. If this fan stops working, the coolant temperature will drop only slightly as it passes through the radiator, which can cause overheating as it passes through the engine.

Broken fan blades, a failed fan motor, faulty wiring, or a failing fan clutch can cause the radiator fan to malfunction. 

If you notice your fan never turns on or spins slower than normal, this may be an indicator your fan has failed. 

4. Faulty Temperature Sensor

On vehicles with an electric fan, the temperature sensor sends power to the fan when the coolant reaches a specific temperature, engaging the fan to lower the coolant temperature. If this sensor fails, the fan won't get the electricity it needs to operate, causing the car's engine to overheat. 

5. Low Coolant Level

With low coolant, the cooling system can't effectively absorb the heat from the engine and cool it. This can cause overheating, though it generally requires severely low coolant before it'll overheat to a point it's noticeable. 

Tell-tale signs of a low coolant level are when the temperature gauge reads slightly higher than normal, but it's not in the overheating range yet, and the coolant doesn't read the "Full Hot" or "Full Cold" marks on the reservoir. 

Cars don't use coolant like they do gasoline. So, if your coolant level is low, this means there is likely a coolant leak. Keep in mind, not all coolant leaks are on the outside -- they can also occur inside the engine. For example, a blown head gasket, which seals the cylinder head to the engine, can cause the coolant to leak into the cylinders or the oil system. 

6. Clogging in the Cooling System

Over time corrosion and other debris in the cooling system can cause buildup that clogs the cooling system passages, especially in cars that aren't maintained properly. This can block the flow of the engine coolant, resulting in overheating. 

7. Airflow Restriction

The radiator fan must pull fresh, cool air through the radiator fins to lower the engine coolant's temperature. Without proper airflow, the coolant temperature won't lower enough, resulting in overheating. A blockage on the radiator can cause this. 

Some common items that block the radiator include plastic bags, leaves, and cardboard. Physical damage to the radiator that has partially or completely sealed the passages through the fins can also cause this. 

8. Low Oil Level

Your car's motor oil is responsible for lubrication, and if the level is too low, this can cause heat buildup. If the excess heat is too great due to a lack of lubrication, the coolant may not reduce the temperature enough, resulting in overheating. 

What to Do When Your Car is Overheating

If you notice that your car's temperature gauge is running higher than normal, or steam is coming out from under the hood, your car may be overheating. It's important to act quickly to avoid damaging your engine. Here's what you should do:

1. Turn Off the A/C and Turn Up the Heat

As soon as you notice your car is overheating, turn off the air conditioning and turn on the heater. It may sound counterintuitive, but this will help draw heat away from the engine and release it through the heater vents, which can help to cool down the engine.

2. Find a Place to Pull Over Safely

It's crucial to find a safe place to pull over as soon as possible when your car is overheating. This will prevent any further damage to your engine and keep you safe. Find a safe spot on the side of the road or in a parking lot, away from traffic.

3. Check and Refill Coolant

After turning off the engine, wait 10-15 minutes for it to cool down before opening the hood. Once the engine has cooled, check the coolant level in the overflow tank. If it's low, refill it with the type of coolant recommended in your owner's manual. Be sure to avoid opening the radiator cap until the engine has cooled completely.

4. Restart Engine

After refilling the coolant, you can restart the engine and monitor the temperature gauge. If it stays within a normal range, you can continue driving. However, if it starts to overheat again, it's best to turn off the engine and seek professional help.

Let Clutch Replace Your Overheating Car

Tired of dealing with the overheating problems your aging car is having? Clutch can help with a quality pre-owned vehicle

All our vehicles endure a 210-point inspection and reconditioning process, including checking the cooling system for any issues. This ensures you get a top-notch pre-owned vehicle at a fair price. Plus, we include a 90-day/6,000km warranty on our vehicles just in case there is an issue. 

At Clutch, we take the stress out of car buying with our 100% online car-buying experience. You choose the car you want online, you secure financing, and we deliver it. You then get a 10-day test-ownership period where you can drive the car and see if it's right for you. If it's not the right vehicle, simply return it for a full refund or exchange it for another.