Your car overheating at idle in heavy traffic or at a stoplight is one of the worst things that can happen to any car owner. Unfortunately, this scenario is quite common. Knowing why this happens is the first step to learning how to solve it.
Car overheating at idle is mostly caused by the radiator cooling fan not working properly or due to a bad coolant temperature sensor for the fan. However, you can’t completely rule out other possible causes for this issue such as a low coolant level, a faulty thermostat, a failing water pump, plugged radiator, or fan belt issues.
Discover more in-depth information below about the causes of your car overheating at idle and what you can do to resolve it. Plus, we’ve shared other helpful tips and tricks to keep in mind when faced with this problem.
Why does your car overheat at idle?
If you’re faced with a situation where your car overheats when idling, but not while driving, then the first thing you need to know is where this problem comes from.
Below are all the possible reasons causes for your car overheating at idle:
Broken radiator fan
A malfunctioning radiator cooling fan is the most common cause of your car engine overheating at idle.
As you drive your car, the airflow over your radiator helps reduce the coolant temperature in your cooling system, so it gets recirculated back to the engine to draw more heat, helping prevent overheating.
This cycle keeps repeating itself as long as you’re driving. But when you get to a stop light or in heavy traffic, there’s not enough forward motion to create airflow across your radiator.
This is where the radiator cooling fan comes into play. It helps blow air over your radiator when you’re moving at a slow speed or at idle, ensuring the cooling system continues functioning well, even at stops.
However, if this fan becomes faulty, probably because the fan motor has been burned out, due to an electrical short that cut off the power supply to your fans, or debris got lodged into the fan, then your engine may start overheating at idle.
But after you resume driving and the airflow is created over your radiator, the temperature will fall back to normal range again, and the overheating stops.
Low or contaminated coolant
As you already know, the coolant’s job is to circulate through your engine, absorbing excess heat from the engine’s hottest moving parts. In so doing, this fluid helps keep your engine running at its normal operating range.
Unfortunately, this fluid can get contaminated over time as it circulates through your engine lines.
It may pick up engine debris like corrosion and other undesired elements which will end up affecting its cooling performance. This will likely make your engine overeat when idling or at a stoplight.
Likewise, if the coolant level goes too low, say due to a coolant leak or simply gets depleted, your cooling system is left with nothing to cool down your engine and prevent overheating.
Another reason your car may be overheating at idle is due to a bad thermometer.
When your car engine is running at its ideal temperature, the thermostat simply opens to allow coolant to flow around the engine to keep the temperature in check.
If this thermostat gets stuck in the closed position or opens only partially, your cooling system may start underperforming.
Your car may be enjoying sufficient airflow when on the move, but on stopping, the engine will get too hot. Installing a new thermostat that opens/closes reliably can help fix this problem.
Bad water pump
If your engine water pump isn’t doing its job as expected, then your car may also start overheating when idling. The water pump simply pumps the coolant through the car engine system
Since this pump is typically belt-driven, giving your car some gas may help improve the cooling but the temperature will rise again once your engine goes back to idle.
Faulty cooling system
Other than the water pump, other parts of the cooling system not functioning correctly will also cause your engine to overheat at idle.
You’d want to check the conditions of various components including the radiator and fan belt.
How do you fix an overheating engine while idling?
When your car starts overheating while idling in light or heavy traffic, you don’t need to panic. Simply find a safe place to pull over and turn off your engine.
Allow the engine to cool for around 15-20 minutes as you keep an eye on the temperature gauge. As the engine cools, the gauge should also move back to normal temp range.
Check the coolant level
Once your engine has cooled down, go ahead and unscrew the radiator cap to inspect the coolant level in the radiator. If the level is too low, top it up as you may have found the culprit behind the idling. Also, inspect your car for coolant leaks.
Check if the fans are working
Next, check if your radiator fans are spinning. Turn your car on and let it hit its operating temperature. Be sure to turn off all climate controls. Leaving these or the heat means bypassing the relay trigger, which is what you’re testing for in this part.
Pop open the hood and visually inspect the fans as they kick on. If your car starts overheating and the fans don’t kick on, then the issue could be the motor controlling the fan or the relay responsible for triggering the fan to kick on.
You can go ahead and test the fan motor to help you rule out whether it’s the motor or the relay that’s problematic.
To do this, simply unplug the motor’s electrical connector so you can see two metal pins inside.
With gloves on, grab two wire lengths and run one of them from the car battery +Ve terminal to one of the pins. Run the other wire from the –Ve terminal to the other electrical pin. If the motor is fine, the fan should spin.
Inspect the thermostat
The best way to test the thermostat condition is by removing it and throwing it in a pot of boiling water. Let it stay for a few moments and then get it out. The thermostat should open when you get it out of the hot water and close slowly as it cools down.
If you still haven’t the cause of the overheating after all the above diagnoses, then you’re probably dealing with more serious issues such as a faulty water pump. While the water pump isn’t hard to replace on your own, you may want to get a professional to do the job for you.
Why your car overheats when idling with AC on
If your car overheats when idling with the AC on, it is most likely caused by a faulty fan. The switch responsible for controlling the fan may not be working and needs to be replaced.
Other reasons why your car overheats when idling with AC on include low coolant levels, a clogged air filter, a faulty thermostat, or a bad water pump.
But what’s the connection between the AC unit and car overheating? You may ask…
The AC compressor usually comes connected to your car engine by a belt that acts like a clutch. Transmission between the engine and AC is magnetic and is usually managed by several low and high-pressure sensors which help keep it moving.
Now, the AC unit adds a significant extra strain on your engine, causing production of heat. As the strain continues, more heat gets generated, resulting in an overheating condition.
This condition can worsen if your AC compressor is overloaded and its belt isn’t functioning correctly. The strain on the engine amplifies and heat production increases.
If your cooling system isn’t able to cool down the engine to any of the issues mentioned above, then an overheating occurs.
Can low oil cause overheating while idling?
Low engine oil can cause overheating of your engine while idling. If you have checked all the potential causes we discussed earlier and still couldn’t find the reason for your car overheating, you may want to check the oil levels.
The truth about your engine oil is that it not only lubricates the moving parts of your engine but it also helps in cooling down these moving parts.
If the oil level goes low, friction increases around these parts. More friction translates to more heat production, and this can be the source of overheating. Make sure your car engine has enough oil!
Can a car overheat from idling too long?
Absolutely! If your car idles for too long, e.g. when you’re stuck in heavy traffic or at a stoplight, then your engine may also start overheating.
Your vehicle engine may start overheating if left to idle for more than 10-15 minutes, especially when driving in hot weather or when your air conditioning is running (or both).
So, next time you find yourself tuck in heavy traffic and are worried about your engine overheating, consider driving your car with a lighter foot on gas to ensure air continues circulating over your radiator.
Also, if your car starts overheating, you’d want to turn off your air conditioner and turn on the heater to cool down your engine.If you’re experiencing overheating issues with your car at idle, it’s important to address the problem as soon as possible to prevent damage to your engine. At MotorAdvices, we have informative articles on a range of automotive issues, including what to do after an oil change and after installing a new radiator to prevent overheating. Our article on car overheating after an oil change explains why this can happen and provides tips on how to avoid it. Similarly, our article on car overheating after installing a new radiator discusses the possible reasons for overheating and offers solutions to help you keep your engine running cool. Visit MotorAdvices to learn more about how to prevent overheating and keep your car running smoothly.
The most likely cause for your car overheating only when driving is the cooling fan failure. When driving, your car has natural air intake which helps keep the engine cool. At idle, this airflow is facilitated by the fan. If the fan isn’t working properly, then your engine will start to overheat.
A blown head gasket can indirectly cause engine overheating at idle by causing coolant leaks. As we said earlier, low coolant level is one of the causes of overheating at idle. If you have noticed a mysterious loss of coolant in your car with no visible leaks, then you probably have a blown head gasket. Though you can check for additional signs of a bad gasket to confirm if this part is indeed faulty.
Yes, if your water pump is dead or has a broken impeller, it’s unable to circulate the coolant throughout your car engine system, increasing the chances of your engine overheating at idle.
There are many possible causes for your car overheating at idle. The most common causes include your radiator fan not working, a stuck thermostat, low coolant level, contaminated coolant, a worn-out water pump, or other malfunctioning parts of your engine cooling system.
You should have your car diagnosed and fixed by a professional mechanic if it starts overheating only when you stop at a light or in heavy traffic. You can also follow the tips diagnosis tips we’ve discussed in this guide to help you find out what’s causing the overheating problem and fix it on your own.