What Causes a Car to Overheat?

It happens at the absolute worst time. You’re headed to the water park with the kids on a stifling summer day and the A/C’s already conked out, or you’re running late and encounter standstill, bumper-to-bumper traffic on the interstate. What causes a car to overheat are conditions just like this.

Few problems with a car are as disruptive as an engine overheating, and both the cost and timeline to repair the issue are almost guaranteed to be frustrating. Here’s what you need to know about the reasons for a car overheating and how to try and deal with it.

Reasons for engine overheating

When an engine’s heat gauge climbs into the red zone, it’s almost always because there’s a problem with the cooling system. It can’t regulate the engine temperature by itself.

Whether you drive an economically priced subcompact car, a behemoth of a pickup truck or a high-revving supercar, it’s designed to withstand normal operating conditions of all kinds, heat- or cold-related. An engine has a temperature “sweet spot” for efficiency. It’s between 195 and 220 degrees Fahrenheit, and exceeding that temperature can have devastating consequences. Here are reasons that temperature could be exceeded:

Low coolant

Coolant or antifreeze is the liquid that circulates in the cooling system. If it’s low, a pocket of air called an “airlock” can sit at the highest point in the engine and prevent the coolant from circulating. When that happens, the coolant temperature continues to climb with the inevitable result being a plume of steam coming from your engine bay.

Having low coolant usually indicates a leak in the system, but not always. Some vehicle owners could find a small amount of coolant evaporates over time, or is consumed by the engine by seeping past the head gasket. That’s why checking the coolant level and other fluids during an oil change is advised.

Water pump failure

The water pump acts as a small turbine that pulls coolant through the cooling system. If the water pump isn’t actively circulating the fluid through the cooling system, then the hot fluid can’t make it to the radiator to cool down, and the cool fluid doesn’t return to the engine to absorb more heat.

Collapsed radiator hose

Again, flow becomes an issue if a radiator hose is collapsed. Coolant can’t move throughout the engine and radiator if a hose is squished together on itself. Sometimes it can be due to the hose itself deteriorating and its layers separating, but it’s most likely because there’s a vacuum condition in the system. In most cases, it’s the radiator cap that isn’t normalizing pressure, but the exhibited symptom is a collapsed hose.

Leaking heater core

If you’re getting the sickly sweet smell of coolant inside your car as the temperature gauge skyrockets, it could be a leaking heater core. This is the component that heats your car’s interior on a cold day and looks like a little radiator tucked up under the dash. A leak here can cause your engine to overheat since a leak means low coolant, and we’ve already looked at what that does.

Thermostat stuck closed

The thermostat is designed to stay closed until your engine reaches its operating temperature, then it opens. A closed thermostat acts as a restriction that helps the engine heat up quickly. But if it doesn’t open, the temperature continues to climb … and climb … and climb. No coolant flow is a bad thing for an engine and it doesn’t take long for a stuck thermostat to cause engine overheating.

Improper coolant concentration

Most coolant mixtures are intended to be 50% coolant and 50% distilled water. If the coolant mixture is too concentrated, overheating can be the result. Water is a much better conductor of heat, so increasing the ratio may protect better against freezing, but can be detrimental since coolant isn’t great at conducting heat on its own.

Plugged radiator

The radiator is intended to transfer heat from coolant passing through it to the ambient air outside it. A radiator that’s plugged or partially restricted inside will prevent the coolant from passing through effectively, holding the hot coolant in the engine for longer and driving temperatures up.

But a radiator can also be plugged externally. Tiny fins help disperse heat, but if they are clogged with dirt and bugs or all dinged up from stones, it will affect the radiator’s ability to radiate heat.

Radiator fan isn’t working

The radiator fan draws air from outside the engine bay across the radiator to more efficiently cool the hot fluid. But if that radiator fan isn’t working, especially when you’re at a stop, it can be the reason for your car overheating. It’s the same whether you have an electric cooling fan or a mechanical clutch fan.

Shredded belt

In many cars, a serpentine belt or V-belt is used to spin the water pump pulley, the fan clutch pulley or both. Belts are made of reinforced rubber, but they can deteriorate and shred or simply snap or come off the pulleys. When that happens, the water pump doesn’t pull coolant through the engine, and the cooling fan won’t draw air through the radiator.

Driving like a maniac

An engine’s combustion chambers control mini explosions several times per minute under normal operating conditions. If you continuously push the bounds of your engine’s performance, be it an ill-prepared pull on a quarter-mile drag strip or hauling down the interstate with an overweight load, the extra stress and added heat from more internal combustion can cause car overheating, even if nothing else is wrong with the engine.

How does the engine cooling system work?

Now you know 10 of the potential car overheating causes, but a background on how the cooling system works could help you identify issues that can come up.

Engine coolant fills the complete system and is most commonly a 50/50 mix of ethylene glycol–based coolant and pure, distilled water. As it passes through galleries or channels in the engine block and cylinder heads, heat is transferred from the denser, hotter metal parts into the lower-temperature fluid.

A thermostat blocks the route through the cooling system until it reaches operating temperature and the thermostat opens. If it drops below operating temperature, the thermostat closes again, then reopens when it warms.

Coolant flows to the radiator where it passes through thin core tubes. This is where the heat is transferred from the hot coolant to the air that passes through the radiator, lowering its temperature before returning for another cycle.

The water pump then pulls the coolant from the radiator and pushes it back through the engine to start the process all over again. Other components are fed coolant as auxiliary systems to the engine cooling system like the heater core, throttle body and a turbocharger or supercharger, for example.

What would happen if you ran an engine without a cooling system, you ask? In just a very short time, perhaps a minute or two after startup, temperatures would exceed operating temperatures and the engine would begin to overheat. With nothing to regulate the temperature, metal parts would begin to warp and the lubricating properties of the engine oil would fail. The engine would either seize or fail catastrophically soon after.

What to do if your car is overheating

Overheating is a serious issue and doesn’t happen without a good reason. The problem doesn’t fix itself either. When your car overheats, it’s best to get off the road and out of traffic as soon as you can. Then, get it looked at. If you find yourself in the middle of a situation when the engine is running up into the danger zone, these are good steps to take:

Turn off the A/C

Sounds ridiculous, right? However, air conditioning adds tons of strain to your engine that can cause temperatures to increase under the hood. Press that little snowflake button on your HVAC controls to turn off the blasting cold air and you might see the temperature gauge begin to descend.

Crank the heat

Again, it seems counterintuitive to turn up the heat in your car. However, what it does is redirect coolant through the heater core, acting as a small secondary radiator that can help disperse heat. It’s going to get hot in the car, so think about opening your windows, too.

Stop driving

Pull over to the side of the road and shut the engine off if you can’t get your engine overheating under control. Ideally, pull over and shut it down before the temperature needle is pinned at the top of the gauge. Then wait and, with any amount of luck, you’ll have stopped the problem before it’s caused lasting damage.

Open the hood

All that heat ruminating under the hood is like a little oven. Don’t do it immediately (you might catch a face full of scalding steam), but once things have cooled off a bit pop the hood and let fresh air get at and around the engine. You may want to use gloves in case the hood latch is still hot.

Let the engine cool on its own

For goodness’ sake, don’t try to accelerate the cooling process. Pouring water or packing ice on hot metal parts can cause them to flash boil and burn you or make them rapidly contract and warp or crack. Just let the air do its thing and cool your engine.

Add coolant

The coolant level is most likely low, and overheating commonly forces even more out of an overflow tube. Once the engine has cooled down—probably hours later, to be honest—crack open the radiator cap and top up the coolant. Use pre-mixed 50/50 coolant that’s designed for your vehicle to top it up. In a pinch, you can use straight water, but you’ll need to adjust the coolant strength later on.

Get it checked out

The problem won’t fix itself. If your engine has overheated once, there’s a solid chance it’s going to happen again unless you do something about it. If you’re handy, figure out what’s wrong and do the repair. If this type of diagnosis and repair is beyond your scope or comfort, take it to a shop and get it dealt with.

How to keep a car from overheating

What can cause a car to overheat is usually avoidable. Like changing the oil, rotating the tires and a host of other measures, an ounce of prevention is worth a pound of cure. Not only will you avoid frustration from breaking down but you can save fistfuls of cash on unnecessary repairs.

• Check the coolant level at regular intervals, or at least every oil change.
• Top up coolant with the same type to prevent corrosion or gelling in the cooling system.
• Change the engine coolant according to your maintenance schedule to prevent buildup and acidity inside the system.
• Park your car in the shade or in a garage to start your trip with a cooler engine.
• Tint your windows to keep the cabin cooler, reducing the amount of A/C you need.
• Open your windows rather than use the air conditioning when possible.
• Get cooling system leaks fixed as soon as they’re discovered.
• Take lower-traffic routes to work, avoiding potential gridlock that can elevate engine temperatures.
• Can you drive through well-shaded areas rather than major routes? It can keep your operating temperature lower.
• Run the heat at the first sign your engine temperature is too high.
• Keep an extra container of pre-mixed coolant with you in your car.
• Get routine car inspections.