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Why is my car overheating? Common causes and what to check
What to do right away when your temperature gauge climbs, the most common reasons cars overheat, and what you can safely check yourself.
A temperature gauge creeping toward hot, or a red coolant warning on the dash, is one of the warnings you should never ignore. An engine that keeps running while it overheats can warp its cylinder head, blow a head gasket, or seize. A repair that might have been a few hundred dollars can turn into an engine replacement.
Here's what to do in the moment, the most common causes, and what you can safely check yourself.
What to do right away
- Turn off the A/C. It adds load to the engine and heat to the radiator.
- Turn the heater to full hot and the fan to high. It feels backwards, but your heater core is a small radiator. Running it pulls some heat away from the engine.
- Pull over as soon as it's safe and shut the engine off. If the gauge is in the red or the warning light is on, don't keep driving to "make it home."
- Don't open the hood right away if you see steam pouring out. Give it a few minutes.
- Call for a tow if you're not sure. A tow is cheap compared with a damaged engine.
Safety first: never open a hot radiator cap
The cooling system runs under pressure, and that pressure lets coolant get hotter than boiling without turning to steam. If you open the radiator cap or the expansion tank cap on a hot engine, that pressure is released all at once. Scalding coolant and steam can spray out and cause serious burns.
- Wait until the engine is cool to the touch. That often takes at least 30 minutes, and longer on a hot day.
- Even then, open the cap slowly with a thick rag over it, and stand to the side.
- Don't pour cold water into a hot engine. A sudden temperature change can crack a hot cylinder head or block.
- Keep away from the cooling fan. Electric fans on many cars can switch on even with the engine off.
- Coolant is toxic and tastes sweet to pets and kids. Clean up spills.
The most common causes
Low coolant from a leak
This is the most common cause. Coolant can leak from hoses, the radiator, the water pump, or a cracked plastic expansion tank or fitting. Plastic cooling parts are common on many European cars and get brittle with age and heat. Signs include a sweet smell, puddles that are green, pink, orange, or blue, and crusty residue around hoses and fittings.
A thermostat stuck closed
The thermostat controls when coolant flows to the radiator. If it sticks closed, the engine can overheat quickly, often while the radiator itself still feels cool. Thermostats are a common wear item and are often replaced along with a water pump.
A failing water pump
The water pump moves coolant through the engine. Pumps can leak from their seal or simply stop moving enough coolant. Some engines, including BMW's N54, use an electric water pump that can fail with little warning. Owners have reported what they paid to replace the BMW N54 electric water pump and thermostat and the VW EA888 Gen 3 water pump and thermostat housing.
Cooling fan problems
If your car overheats mostly in traffic or at idle but is fine on the highway, suspect the cooling fan. At speed, air moves through the radiator on its own. When you're stopped, the fan has to do that work. A failed fan motor, relay, fuse, sensor, or (on older cars) fan clutch can all cause this pattern.
A clogged or damaged radiator
Over time, radiators can clog inside with deposits or outside with bugs, dirt, and debris. Bent fins and internal clogs both cut cooling capacity.
Air in the cooling system
After any cooling system work, air can get trapped in the system. Many modern cars, especially BMWs, need a specific bleeding procedure to get it out. A car that starts overheating right after a coolant job often just has an air pocket.
A bad radiator or expansion tank cap
The cap keeps the system at the right pressure. A worn seal lets pressure escape, which lowers the boiling point and can let coolant boil away.
Head gasket failure
A failing head gasket can push combustion gases into the coolant, or let coolant leak into the cylinders. Signs include white exhaust smoke with a sweet smell, bubbles in the expansion tank, coolant loss with no visible leak, and milky oil. Some engines are known for it. Subaru's non-turbo EJ25 is a well-known example. See what owners paid for Subaru EJ25 head gaskets.
What you can check yourself (engine cold)
- Coolant level. Check the expansion tank against its min and max marks.
- Visible leaks. Look under the car and around hoses, the radiator, and the water pump area. Look for wet spots or dried crust.
- Hoses. With the engine cold, squeeze them. They should feel firm, not mushy, cracked, or swollen.
- The fan. Let the car idle until it's warm and watch whether the fan comes on. If you have A/C, many cars will run the fan when it's on.
- Exhaust and oil. Look for white, sweet-smelling smoke and check the oil cap and dipstick for a milky, light-brown color.
- Warning lights and codes. Many cars store fault codes for fan, thermostat, or electric water pump problems. A shop can read them, or you can use an inexpensive scanner.
If you top off coolant, use the type your manufacturer specifies. Mixing types can cause problems. Adding plain water in an emergency is better than running dry, but have the mix corrected afterward.
When to stop and get help
If the car has overheated badly, keeps losing coolant, shows signs of a head gasket problem, or overheats again after you top it off, stop driving it and have it inspected. A shop can pressure-test the system to find leaks and test for combustion gases in the coolant. Our guide on how to tell if a repair quote is fair can help when the estimate comes in.
This article is general information for car owners. It is not a substitute for a diagnosis by a qualified mechanic who can inspect your car. Prices mentioned are medians of owner reports, not quotes.