The Most Common Causes of Car Overheating
Why your car overheats and what to do about it
Quick Summary
The most common causes of car overheating include low coolant, radiator fan failure, thermostat failure, water pump damage, and radiator clogs. If your car overheats, pull over safely, shut off the engine, and call a tow truck. Continuing to drive can destroy the head gasket, costing thousands to repair.
Article
As a master technician who has diagnosed hundreds of cases of car overheating, I can tell you this: engines rarely overheat without warning signs. When drivers understand the most common causes of car overheating, they can prevent major engine damage long before it happens. In this guide, I’ll walk you through what to do if your car overheats, why it happens, and how to prevent it from happening again.
A low coolant level is the most common reason a car overheats
A properly operating engine doesn’t use up coolant. It’s a closed system that continually recirculates the same coolant. If the system develops a leak, the lowered coolant level can cause your engine to overheat.
How a cooling system loses coolant
• Worn, cracked, or burst rubber hoses— Rubber coolant hoses crack and deteriorate over time. When they fail, they leak coolant.
• Rusted metal coolant lines— Modern vehicles incorporate metal coolant tubes that can rust and leak over time.
• Cracked plastic fittings— Carmakers often use plastic connectors for mating rubber and metal cooling lines to the engine, radiator, and heater core. Over time, the plastic fittings degrade, crack, and leak coolant
• Water Pump Failure—The water pump circulates coolant throughout the engine. It uses a ceramic seal on the shaft. Using the wrong coolant or allowing debris into the system can cause the seal to wear and leak coolant.
• Head gasket failure— A breach in the head gasket that occurs between a coolant passage and the cylinder can cause the engine to burn coolant, resulting in an engine overheating condition.
Radiator fan failure is another reason why engines overheat
The radiator cooling fan draws air through the radiator to dissipate heat. If the fan fails, the radiator can’t effectively cool the engine. Carmakers use several types of fans, and each can fail uniquely.
• Single-speed electric fan—Carmakers often use two single-speed radiator fans. In normal operation, one fan runs to keep the engine at normal operating temperature. If the vehicle is equipped with air conditioning, the second fan runs when the AC is on to add additional airflow through the AC condenser and the radiator. If the AC isn’t running, but the ECM determines the single fan isn’t producing enough airflow, it activates the second fan. If a fan fails, the engine overheats
• Two electric fans wired to each run at two selectable speeds— Some carmakers install two electric fans and vary their speed using relays that cause the power to run in a serial or parallel path. See the GM relay setup below. Relay failure is the most common cause of engine overheating in this system.
• One or more infinitely variable-speed electric fans— In this design, the carmaker supplies pulsed power to the fan. These systems use a solid-state power transistor to switch the power on and off. Varying the ON time versus the OFF times determines the fan speed. A failure in the solid-state speed controller is the most common cause of engine overheating in variable-speed fan systems.
• A belt-driven fan with a thermal or electric-viscous clutch— On some engine designs, carmakers install a belt-driven fan that varies its speed through a clutch assembly. The clutch allows the fan blade to slip, slowing its speed when less cooling is needed or eliminating slip when maximum cooling is needed. A failure in the thermal or electrical component causes the fan to slip, resulting in engine overheating constantly.
Radiator Clogging: A Frequent and Overlooked Cause of Car Overheating
Radiators can clog internally or externally, and
Radiator fins clogged with bugs and dust
both contribute to the most common causes of car overheating.
Internal Clogging — Neglected coolant becomes acidic, corroding the small passages and preventing heat dissipation.
External Clogging — Dirt, dust, insects, and debris pack the fins, reducing airflow and heat dissipation.
Thermostat Failure: Small Part, Big Car Overheating Problem
A thermostat stuck closed prevents coolant from circulating to the radiator, making it one of the simplest but most common causes of car overheating. Replacing the thermostat usually solves the problem immediately.
The thermostat regulates the flow of coolant 
between the engine and the radiator. If it fails, it can cause overheating.
Water Pump Wear: Hidden Damage That Causes Car Overheating
A water pump’s impeller can erode from cavitation or corrosion. When it loses its ability to circulate coolant, the result is predictable: the car overheats, especially at high RPM.
Notice the fully eroded impeller in this water pump. The loss of coolant flow causes engine overheating.
Cooling System Electrical Failures: Relays, Controllers, and Bad Designs
Some manufacturers make design choices that directly contribute to the most common causes of car overheating.
GM uses three relays to control its radiator fans, allowing them to run at two or three speeds.
GM uses multiple relays to control fans at low, medium, and high speeds. If even one relay fails, the high-speed fan never engages—leading to car overheating during heavy load or AC use.
GM 2-speed radiator fan wiring diagram
Radiator fan wiring diagram for 3-speed fans
GM 3-speed radiator fan wiring diagram
Chrysler Digital Fan Controllers
Chrysler mounted the controller low in the engine bay, where road salt and moisture corrode the heat sink. When the controller overheats and fails, the car overheats almost immediately. 
If you replace the relay without removing the corrosion, you’ll have another failure. To do the job right, remove the corrosion and coat the cleaned frame member with dielectric grease before replacing the relay.
Viscous Fan Clutch Failures: Mechanical Causes of Car Overheating
Older fan clutches use a bi-metallic spring to regulate silicone fluid pressure. When they fail, the fan freewheels—offering little to no cooling at idle.
Newer electronically controlled viscous clutches (especially early GM versions) are notorious for failures that cause the car to overheat when stopped.
What to Do If Your Car Overheats — Expert Steps
When I teach drivers what to do if their car overheats, I emphasize safety and immediate damage prevention:
1) Pull over safely and shut off the engine — You’ll find advice online to keep the engine running and turn your blower motor to heat and high fan speed. That’s bad advice. It may have worked back when all engines were made from cast iron, but it’s horrible advice for modern aluminum engines. Letting an overheated engine run is certain to cause cylinder head warping and gasket damage.
2) Call a tow truck — Yes, that costs money, but continuing to drive with an overheating condition will cause far more damage. The tow is always cheaper.
Knowing what to do if your car overheats can save your engine from catastrophic failure.
© 2016 Rick Muscoplat
Posted on by Rick Muscoplat