What Causes a Car to Overheat?

Imagine you are driving down the road, listening to your favorite song, and all of a sudden your car begins to sputter, you hear a warning chime, and the Check Engine light or temperature warning light comes on. As you pull to the side of the road, steam starts coming from under the hood, and you notice the needle on your temperature gauge is pointing to hot. You instantly realize your car is overheating. But what could have caused your car to overheat?

A car overheats when the engine temperature rises to a higher level than the designated temperature specifications. Overheating usually happens when there is too little coolant in the cooling system or if the coolant is not circulating correctly through the cooling system. Major engine damage can occur depending on how hot the engine gets and how long it stays overheated. Overheating can cause the head gasket to fail, cylinder heads or the engine block to warp or crack, or in extreme cases, the engine oil can break down causing pistons to crack or the engine to seize. So, overheating issues need to be addressed immediately.

sealant cap for engine coolant

If your car overheats, the first thing to check is the coolant level. Once the engine has cooled, you can remove the radiator cap or coolant reservoir cap to check the coolant level in the system. In some cases, the coolant reservoir may provide a marking to indicate the appropriate coolant fill level if the cooling system is hot or cold. You should never remove the radiator cap while the engine and radiator are hot or overheated. The coolant will be under pressure, and removing the cap could cause the coolant to spray and cause severe burns on your hands, arms, or face.

When you remove the radiator cap, the coolant level should be at or near the top of the radiator. You will need to add coolant if the radiator is not full or if the radiator appears to be empty. Concentrated coolant needs to be mixed with distilled water to properly cool the engine, so you may need to find the correct mixture ratio in the owner's manual or repair manual for your vehicle. You can purchase concentrated or premixed coolant from your local O'Reilly Auto Parts store.

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Once the radiator is full, you can start the engine with the radiator cap off. Allow the engine to run for a few minutes and cycle coolant through the system, but shut the engine off immediately if coolant begins to flow out of the radiator. If the coolant level drops while the engine is running, shut the engine off, refill the coolant, and replace the radiator cap. Then restart the engine and watch to make sure the engine comes up to operating temperature but does not overheat.

pouring coolant into an engine

If the radiator is full of coolant and your vehicle continues to overheat, there is likely an issue with coolant circulation. There are multiple reasons coolant may not be circulating correctly throughout the cooling system. Some issues can be diagnosed by looking at engine components, but other issues will require parts of the cooling system to be disassembled for inspection.

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Below are areas where cooling system issues can occur. Remember, hot coolant can cause severe burns, so do not work on or disassemble parts of the cooling system while the engine is hot. You should also try to avoid spilling coolant on the ground, so make sure to have a catch pan under your vehicle for coolant that drains from the system. You may need to consult the repair manual for your vehicle to ensure you know the proper steps required to complete these repairs or inspections. Learn more about cooling system basics with our helpful, detailed article.


  • The water pump is how the coolant is circulated through the coolant system. If the impeller on the pump fails, the coolant is no longer circulated. Most belt-driven water pumps have a weep hole that starts to leak fluid when the impeller starts to fail. If there is coolant leaking from the water pump, this indicates that it may be failing and may need to be replaced.

  • Leaking coolant hoses can quickly cause the coolant level in the radiator to drop. Inspect the radiator hoses and heater hoses when the engine is cool to see if any of them are leaking. Leaks can occur at or near the ends where the hoses are clamped, or small pinholes can develop and leave discoloration on the hose around the leak. If you find a leaking hose, drain the coolant out of the radiator and hose. Then, replace the hose by removing the hose clamps, detaching the leaking hose on both ends, and attaching the new hose by tightening the hose clamps on each end. You can then refill the radiator with coolant, start the engine, and check for leaks.

  • The thermostat regulates the temperature of the engine so that it operates at the optimal temperature. Thermostats have a spring-loaded piston valve that opens when the coolant reaches a specific temperature, allowing coolant to flow through the engine until the thermostat is opened, and then flow to the radiator for cooling when needed. If the thermostat fails to open, coolant will not flow to the radiator and the engine temperature will continue to rise. If you suspect the thermostat has failed, it should be replaced immediately.You likely cannot see how the thermostat has failed unless the spring has broken. The thermostat is normally found inside a housing where the coolant exits the engine and is installed against the engine block, timing cover, or intake manifold. To replace the thermostat, you will need to drain the coolant from the hose connected to the housing and possibly drain the radiator. After all the coolant is drained, you can remove the housing. You will then need to clean the old gasket material off the housing, place the new thermostat in the engine, bolt the housing back on the engine with a new gasket and sealant, if needed, and reattach the coolant hose. You can then refill the radiator with coolant and start the engine. Watch the engine temperature until the thermostat opens and ensure the temperature stays at the appropriate operating level.

  • The radiator can clog over time because of impurities or minerals in the system. If the coolant level is low, you may see clogged or gummed-up cores. Coolant cannot be cooled if it can't flow through the radiator cores. In some cases, a radiator flush or cleaner can be used to clean the cores. Different types of coolant flush may have different directions, so follow the directions on the coolant flush container to clean the radiator. The radiator may also be damaged or develop pinholes due to corrosion or impact. This type of damage normally has to be repaired by replacing the radiator or taking the damaged radiator to a professional shop that specializes in radiator repair.

  • Loose serpentine or V-belts may slip and cause the water pump not to turn, or to turn less than it needs to properly circulate the coolant. The belt that turns the water pump may squeak when loose. It may also show signs of stretching, like cracks in the grooves or inside of the belt. The belts may also break, causing all of the engine accessories, like the water pump, alternator, or air conditioning, to stop working. Broken belts should be replaced immediately, and worn belts should be replaced at least as often as the manufacturer-recommended interval suggests.

  • The radiator cooling fan can stop working, and it will not move air through the radiator when your vehicle slows or stops. Depending on the make and model, your vehicle will have either an electric or belt-driven cooling fan, which may also include a fan clutch. If your vehicle has an electric fan, the fan motor, temperature sensor, or fan relay may go bad or the fan may also quit working if it blows a fuse. If your vehicle has a belt-driven fan, it may stop working if the fan clutch fails or the belt breaks.

We hope this has helped you understand why your vehicle may overheat and how to diagnose the problem. You may find that there is more than one cause to your vehicle's overheating issue. So, if you are unable to figure out why your car is overheating or aren't comfortable completing the repairs, we can provide a local shop referral for you to get your vehicle repaired and back on the road.


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