Why Does My Car Keep Overheating?

Summer heat increases the danger of your car overheating, but your engine can run hot at any time. Learn the most likely causes and fixes.

The Most Common Causes of Cars Overheating

Engines can overheat for many reasons. Usually, the cooling system has a problem releasing heat generated by the engine to the outside air. Hot coolant flowing through the radiator should be cooled as air from the cooling fan(s), or normal driving, passes over the radiator.

If your car overheats once, it’ll do it again. An engine overheating can affect drivability and air conditioner efficiency. Ignoring the temperature warning light or a temperature gauge that’s in the red can seriously damage your engine.

Engine Coolant

Engine coolant, a 50/50 mix of water (distilled H2O is best) and anti-freeze, circulates throughout the cooling system to help maintain proper engine operating temperature. Over time, coolant and its rust, corrosion and lubricating properties break down and deteriorate. This enables contaminates such as rust, scale and acids to build-up, clogging coolant system passages.

A clogged cooling system, along with contaminated coolant, severely impair the cooling system’s ability to absorb heat and expel it through the radiator. Flushing your cooling system to remove and neutralize contaminates can be a DIY endeavor, or you can have it done at a repair shop.

Engine Coolant Leaks

Low engine coolant due to a leak is one of the main causes of engine overheating. If you’re always adding coolant, check the radiator, thermostat housing, radiator and heater hoses, water pump and coolant reservoir for leaks. Small coolant leaks can evaporate before you see dripping fluid on the ground and often leave whitish/greenish stains that can help identify the leaking part.

Replacing a leaky coolant reservoir and most radiator and heater hoses and clamps are DIY repairs. Check your owner’s manual to see if replacing other cooling system parts is DIYable.

Thermostat

The thermostat is a simple but critical valve that blocks coolant from circulating through the cooling system when an engine is cold, allowing the engine to heat up quickly. Then the thermostat opens, letting the coolant flow when the engine reaches operating temperature. A malfunctioning thermostat that fails to open prevents coolant from circulating and will cause your engine to overheat, even as you drive.

On some vehicles you can replace the thermostat yourself — check your owner’s manual. Otherwise, your mechanic can do it.

Radiator

Scale and corrosion caused by contaminated coolant can clog internal radiator passages and cause your engine to run hot. Leaves or dust stuck in your radiator also prevent air from passing over the cooling fins, thus trapping heat. A defective radiator cap that cannot hold pressure can also cause overheating.

Replacing the radiator cap and flushing and filling the radiator with new, fresh coolant is DIYable. So is using a garden hose to flush dirt and debris from the radiator cooling fins.

Safety Reminder: Always let the engine completely cool before removing the radiator cap.

Radiator Cooling Fan

Malfunctioning cooling system fans(s) will cause the engine to overheat, especially while idling. To quickly reach operating temperature, most pre-1990s cars use a fan belt-driven mechanical clutch fan that freewheels when cold, letting coolant warm up faster. Then it engages as the engine heats, forcing air over the cooling fins, dispersing heat. A defective clutch fan that always freewheels will cause overheating. Let your mechanic diagnose and repair this one.

Newer cars use one or more electric cooling fans to push air over the radiator to remove heat from coolant. The engine control module (ECM) uses input data from various sensors to turn the fan(s) on and off. Note: Electric cooling fans can turn on if the engine is not running. Due to the number of sensors, switches and relays that make up the electric cooling fan circuit, diagnosing problems with this system is best left to the experts.

Gaskets

If you keep adding coolant but don’t see fluid puddling under the car, check the tailpipe for thick white smoke or a sweet pungent odor. These symptoms indicate coolant is burning in the combustion chamber due to a bad head gasket, intake manifold gasket or other failed internal engine part. Replacing internal engine gaskets is not a DIY project.

Water Pump

Any issue with a water pump can quickly lead to an overheating engine. Common problems include leaks and damaged or eroded impeller blades that cannot completely circulate engine coolant. Check your owner’s manual to see if replacing your water pump is DIYable.

Heater Core

A plugged heater core (similar to a small radiator) restricts coolant flow and gives off little or no heat from the heater. A leaking heater core produces a slimy film on the windows. Both issues will cause your engine to run hot.

Installing a new heater core can be expensive. You can try flushing a clogged heater core with a garden hose or temporarily bypass a leaking heater core. Although highly discouraged, bypassing a leaking heater core won’t hurt your engine. However, if you bypass the heater core, make sure to replace it before winter arrives. More importantly, a bypassed heater core cannot help draw heat from an engine that is overheating. Replacing a heater core should be done by a pro.

Following your vehicle manufacturer’s preventive maintenance schedule will help extend the life of your car’s cooling system. Correcting an overheating condition can be a minor fix or it may require a more complicated, professional repair.

Robert Lacivita
Bob Lacivita is an award-winning auto technician and career and technical educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics, as well as writing state, federal and organizational foundation grants, and helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into technical/vocational training, for more than 20 years. His work has been featured in Family Handyman, a Reader's Digest book and Classic Bike Rider magazine, among others. Bob and his wife lived through 20 years' worth of DIY home remodeling while parenting two (now grown) boys and now relax by watching their three fabulous granddaughters.