Hey, Why is My Car Overheating?

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Four common causes for vehicle overheating and their related parts.
Your vehicle's engine generates a lot of heat during normal operation. The heat is a good thing when the temperatures are cool outside for the driver’s comfort. But when the temperature gets too hot, the engine in your vehicle starts to suffer. Your cooling system needs to be properly maintained to operate properly to help the longevity of your engine and keep your vehicle safe as it motors down the road.

Is Your Car Overheating?

An overheating car sends obvious signs of danger: steam from the hood, foul smells, and the frightening sight of your thermometer in the red zone.

Automobiles are designed with the engine's cooling system to be effective. Engines run off flammable fuel, motioning various parts at incredible speeds. As a result, they generate tremendous heat in the process and need to be cooled to run safely.

Your vehicle has indicators to warn you of an overheating engine, like the engine temperature gauge symbols on your dashboard. Still, even the indicators won't run well if your car's cooling system is working badly.

Your vehicle's systems for regulating engine temperature include fans that run through the engine, the radiator, and the air conditioning system that vents into the interior. This means that engines overheat when heating or AC systems develop an internal temperature control problem.

Several issues might lead to overheating, but they're all serious problems. First, overheating vehicles are flat-out unsafe to drive. Fortunately, there are resources at a technician's disposal to find the trouble's root cause.

Let's Get To The Bottom Of It!

  • Q: Does your vehicle heat up rapidly after you start it?

    You may also see the check engine light on the dashboard and coolant leaking under your vehicle.

    Your vehicle's thermostat regulates engine heat to keep it consistently at the proper operating temperature. It does this by restricting or pulling the coolant flow through the radiator. If the thermostat fails, it gets stuck in an open or closed position rather than allowing a free flow. If stuck closed, the superheated coolant prevents the engine from leaving, leading to rapid overheating.

  • Q: Does your vehicle get hot after you have been driving for a while?

    You may also hear a squealing noise from the belt and have trouble steering as it heats up.

    Your vehicle's water pump circulates coolant through the engine, thermostat, and radiator. This transfers the heat from the engine's combustion process to the radiator. If it fails to circulate the coolant, the engine will heat up. However, since the thermostat is still working, some coolant flow will continue so that the engine won't overheat until after a bit of driving.

  • Q: Do your windows fog up even with the defroster on?

    You might feel the floor under the passenger's seat is wet and notice that your car's temperature gauge on the dashboard is in the red zone.

    The heater core is a small air box that pulls heat from the coolant into the vehicle's interior. If the heater core leaks, coolant gets in the air box. When hot air flows over the liquid, condensation forms on your windows, making them fog up.

  • Q: Did your vehicle become hard to steer before it overheated?

    You may also have noticed that the heat didn't work well and that the charging or check battery light has been illuminated on the dashboard.

    The serpentine drive belt operates accessories attached to the front of the engine. On some vehicles, it runs the water pump. If the belt fails, all the accessories it drives will also fail. The first thing you'll notice is difficulty steering due to the loss of the steering assist system. You may shortly see the vehicle overheating because the water pump no longer circulates coolant through the engine and radiator.

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Vehicle Health Inspection Proof

Explore the following typical inspection results that show a potential cause for the symptom and select the one you believe is similar to your vehicle's issue.

Coolant low

The proof image shows the leak from the radiator, resulting in low coolant levels.

A customer dropped off their car concerned that the heater no longer worked well. They said it seemed warm when driving at highway speeds but cooled off almost entirely when they hit stop-and-go traffic. They said that they checked to make sure that the air conditioner wasn't on, and that it wasn't, but it felt almost like it was constantly on.

The technician felt the heater was not producing much heat on the test drive.

However, during the vehicle health inspection, the technician noted that the coolant was low and that the radiator was wet from the upper seam, where the core and the tank are crimped together.

The technician performed a cooling system pressure test and verified that the upper tank on the radiator was leaking at the crimp and found no other leaks. The technician recommended replacing the radiator.

Does the issue look like this? if not accessible your shop will document it
Thermostat failing

The proof image shows a closed thermostat. In this case, it shouldn't be closed when the coolant is at the proper temperature. So it's damaged.

A customer dropped off their vehicle after they noticed that the temperature gauge almost reached the red zone after they just barely got out on the highway. The customer drove to a nearby convenience store to stop the overheating vehicle. Since they heard a gurgling under the hood, they decided not to open it. They also saw coolant leaking onto the ground. The customer opted to stop driving the vehicle, so they called a tow truck to bring the car to the shop.

The technician did not do a test drive since they were worried about the incident described by the customer.

However, during the vehicle health inspection, the technician noted that the coolant was a little low and that the only signs of a coolant leak were coming from the overflow reservoir.

After the customer authorized diagnostics, the technician topped off the coolant and pressurized the cooling system with a pressure tester. Then they pumped up the pressure to 12 psi, the pressure stated on the radiator cap. Again, the technician found no external leaks, and the pressure was held.

The technician then started the vehicle and let it idle to warm the coolant. The technician used an infrared thermal reader to check the temperature before the thermostat and then again after the thermostat. The engine came to the operating temperature of 195°, which is the opening temperature of the thermostat. The temperature after the thermostat remained at 80°, while the temperature before the thermostat on the engine side kept rising to 210 °.

The technician shut the vehicle off and relieved pressure from the radiator. The technician then removed the thermostat housing and verified that the thermostat was stuck. As a result, the thermostat and gasket will need to be replaced.

Does the issue look like this? if not accessible your shop will document it
Heater core leaking

The proof image shows coolant leaking from the heater core, eventually causing the car windows to fog up.

A customer dropped the vehicle off after noticing that the defroster was not working. They said the front window is always foggy, even with the defroster on high. In addition, the customer wondered if the air conditioner was not working, as they were aware that it draws the humidity out of the cab's interior during the defrost cycle.

On the test drive, the technician noticed the smell of antifreeze in the vehicle and that the windows had fogged up. However, the technician saw nothing about the customer's concerns during the vehicle health inspection.

After the diagnostics were authorized, the technician used a coolant system pressure tester and pumped the pressure to 12 psi. The technician then inspected the engine compartment and saw no leaks but noticed that the pressure was slowly falling on the tester, designating a leak somewhere.

The technician then gained access to the underneath of the dash, where the heater core was, and found coolant dripping from the airbox. So, the heater core is leaking and must be replaced.

Does the issue look like this? if not accessible your shop will document it
Water pump failing

The proof image shows the broken impeller of the water pump, preventing the pump from circulating coolant.

The customer dropped off their vehicle after they noticed that it sometimes overheated. They said the car didn't appear hot when they drove their kids to school a block away. However, when they go to the grocery store about 5 miles from the house, the temperature gauge is almost at the red line when they get home.

Due to the overheating condition stated by the customer, the technician did not perform a test drive. During the vehicle health inspection, the technician did not notice anything about the customer's concerns.

After additional diagnostics were authorized, the technician used a cooling system pressure tester to test for leaks. The tester was pumped up to 12 psi; no external leaks were found, and the pressure was held. The technician then started the vehicle and used a block in the radiator cap to see if the head gasket had failed, which would cause the blue fluid to turn yellow.

The technician started and ran the vehicle and ran the block test, which did not fail. The technician then used the thermal imaging scanner and found that as the car warmed up, the thermostat opened and went up to 195 degrees.

The technician then removed the bypass hose from the degas bottle to verify that the water pump was pumping and to ensure no flow was coming from the hose. The technician then got authorization to remove the water pump and found that the water pump impeller had broken off and was no longer attached to the pump assembly. Therefore, the water pump will need to be replaced before the car engine overheats more.

Does the issue look like this? if not accessible your shop will document it
Cooling fans failing

The proof image shows the cooling fan. The right red arrow points to where technicians usually perform tests for command voltage readings.

The customer dropped off their vehicle after noticing it ran a little warmer than usual. They said that whenever they slow down or come to a stop, the car temperature gauge rises.

On the test drive, the technician noted that the vehicle's temperature gauge rose at idle, and when they started to drive faster, the temperature gauge continued to rise a little. The technician also noted that the check engine light was on.

However, the technician noticed nothing about the engine overheating during the vehicle health inspection. So, the customer approved additional tests.

The technician performed a code scan and found a code signifying a cooling fan control circuit malfunction. After the customer authorized further diagnostics, the technician used a vehicle-specific scanner to command the electric cooling fan to turn on. However, the cooling fan failed to energize.

The technician then used a digital voltmeter to test the command voltage at the connector to the cooling fan. There was battery voltage at the connector, which proved that the cooling fan motor had failed and would need to be replaced.

Does the issue look like this? if not accessible your shop will document it
Drive belt failure

The proof image shows the drive belt is missing due to a failed tensioner, causing stiff steering and overheating. A car overheats in such cases.

A customer dropped off their vehicle after hearing a squealing noise while getting off the freeway. Once the squealing noise stopped, the car was tough to steer. Then, while waiting at the light at the bottom of the off-ramp to the highway, the temperature gauge started to climb.

When the vehicle was maneuvered into the gas station, the temperature gauge was on the red line. They immediately shut the car off and called a tow truck.

The technician did not perform a test drive; they just drove the vehicle into the service bay. They verified that the power steering was not working and the car was hard to steer.

During the vehicle health inspection, the technician noted that the serpentine drive belt was missing and that the drive belt tensioner pulley had seized and melted.

The technician did not notice any coolant leaks. The technician recommended replacing the serpentine belt tensioner and serpentine belt. After repairs were made, the power steering and cooling systems operated normally.

Does the issue look like this? if not accessible your shop will document it

Typical Fixes to Address the Cause(s)

The following chapters bases themselves on experiences from our auto repair shop; we'll describe related problems' causes and fixes.

"Radiator Cooling Fan Replacement" fixes "Cooling Fans Not Working"

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Hey, Why is My Car Overheating?
Sometimes a problem is more challenging to describe than it initially looked like. If you are not sure your problem is described by this article, please find below similar vehicle symptoms, which might describe better the issue you are experiencing.

Other things your auto repair shop might talk about:

Coolant reservoir tank. Coolant hoses.