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My car is overheating

My car is overheating — What to check

My car is overheating, what could it be is a common question on car forums. Car overheating can be caused by many things. Your car can overheat when it’s stopped at a red light or stop sign, when driving at slow speeds, or your car can overheat at highway speeds. Let’s start with the scenario where your car overheats when stopped

My car overheats when stopped

Low coolant level can cause overheating when stopped situation

coolant reservoir cap for pressurized coolant recover tank

Many late-model vehicles have a pressurized coolant recovery tank, also called a degassing unit. Never open the cap when the engine is hot

You may have just enough coolant in your engine to keep it at an acceptable level when the water pump is spinning at high speeds while driving. But when you stop for a light or stop sign, a low coolant level can cause overheating. There just isn’t enough coolant in the system to cool all the internal components.

That low coolant level may be enough to cool the engine when the water pump it turning at a higher speed and forcing the less-than-optimal amount of coolant around the engine, but the pump simply isn’t spinning fast enough to create enough circulation at low RPMs. Check the coolant level by looking at the level in the coolant reservoir. NEVER open the cap when the engine is hot.

Inoperative radiator fans can a “my car is overheating condition”

Many cars use multiple speed or variable speed radiator fans. So the radiator fan may be running, but not at the right speed. Here’s the rule of thumb. If your car has two radiator fans, BOTH should run at high speed any time your A/C is on. If your A/C is off and you’re at a stop, the radiator fan should kick into high speed as the engine coolant temperature rises.

GM uses three relays to make its radiator fans run at two or three speeds.

Many GM cars use three relays to make the radiator fans run at two speeds. If just one relay fails, the fans can’t run at a high speed. See this wiring diagram.

car overheats radiator fans

GM 2-speed radiator fan wiring diagram

Radiator fan wiring diagram for 3-speed fans

car overheats, radiator fan fuses, fan relay

GM 3-speed radiator fan wiring diagram

Chrysler uses a digital fan controller to regulate radiator fan speeds

The Chrysler fan control gets speed commands from the PCM and changes the radiator fan speeds based on the temperature of the coolant. This “cooling fan relay” requires a good ground and a “heat sink” to keep them cool and prevent failure.

In a total lack of foresight, Chrysler mounted the cooling fan relay on a frame member near chrysler uses a solid state fan relay to provide variable fan speeds for the radiator fanthe bottom of the engine compartment where it’s exposed to road salt and water. The back of the relay is a heat sink, so it must touch the metal frame to dissipate heat buildup. When the frame rusts, the relay overheats and fails.

If you simply 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.

A failed mechanical radiator fan clutch can cause car overheating

Carmakers sometimes install a viscous clutch on mechanically driven radiator fana viscous radiator fan clutch is used for mechanically driven radiator fans. The cluth allows the fan to free-wheel at highway speeds to reduce engine drag, but drives the radiator fan at slower speeds.s. The clutch allows the fan to free-wheel at highway speeds to reduce engine drag. The use of a viscous clutch eliminates that drag. The clutch contains a silicone fluid that makes the connection between the pulley and the fan when the engine is operating at lower RPMs and disengages at higher RPMs.

What goes wrong with a viscous fan clutch?

Older viscous fan clutches have a bi-metallic spring that regulates silicone pressure. When that system fails, the fan goes into free-wheeling mode at all times. So you’ll get some cooling at highway speeds, but the car can overheat when stopped. Replacing the radiator fan clutch solves that problem.

Newer cars and trucks use an electronically controlled viscous fan clutch. GM, for example, uses an electronically controlled fan clutch. Unfortunately, the early models had a very high failure rate. If you have a mechanically driven radiator fan equipped with an electronically controlled viscous clutch and your car overheats when stopped, suspect a bad clutch.

A clogged radiator can cause overheating when stopped

At higher speeds, a clogged radiator may be able to flow and cool enough coolant to keep your engine cool. But if your car overheats when stopped, there’s a good chance the radiator is clogged.

Check the radiator fins for clogging

Shine a flashlight down the fins on the grille side of the radiator. Bugs, road dirt, and corrosion can prevent proper airflow. If the radiator is clean but the car has A/C, check the condition of the condenser coil since it sits in front of the radiator. If you see buildup on the fins, rinse with a garden hose nozzle until clean.

Check the radiator for internal clogging

The easiest way to check the radiator is to use a non-contact infra-red thermometer. Scan across the radiator. Clogs will show up as cool spots, while the rest of the radiator will be hot.

My car is overheating at highway speeds

A failing water pump can cause the engine to overheat

The fins on the water pump impeller may be worn and pump enough coolant at slower speeds but then create cavitation at higher speeds. Cavitation is the formation of air bubbles behind the impeller blade causes by low pressure. The cavitation introduces air bubbles throughout the cooling system, which reduces cooling efficiency, The cavitation can also wear away the metal surfaces in the water pump, further reducing pump efficiency. While a worn pump can sometimes cause overheating at highway speeds, it also causes overheating at slow or stopped conditions as well.

My car is overheating due to missing seals and shrouds

Late-model cars have rubber and foam seals around the radiator to prevent air from bypassing around the radiator. If you’ve had radiator repair work done and the seals aren’t put back in the same place or are left out, air can flow around the sides of the radiator and reduce cooling. The same applies to radiator fan shrouds. Then must be intact and installed properly to create the proper airflow when the vehicle is running at highway speeds.

A failed viscous clutch can cause the engine to overheat

A viscous clutch is designed to allow the mechanically driven radiator fan to free-wheel at highway speeds. If the clutch fails and the fan is still being driven by engine speed when the vehicle is at highway speeds, the fan may actually be running slower than the ram air coming in through the radiator. In that case, the radiator fan can slow airflow and cause a car overheating condition.

a failed head gasket can cause the engine to overheat

If a head gasket fails between a cylinder and a coolant passage, the engine can force hot exhaust gasses into the coolant, substantially raising the temperature of the coolant to the point where it can’t be cooled enough by the radiator. See this post for help diagnosing a head gasket failure.

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