Car Overheats While Idling But Not Driving? Try This Solutions

Car overheating is the last thing you would expect when idling. However, there are situations where the vehicle may start to heat up every time you idle.

But why does your car overheat while idling but not driving? The cause is related to an issue in the cooling system. It could be either of the following problems:

• Low coolant
• Faulty coolant temperature sensor
• Radiator cooling fan issues
• The thermostat is stuck closed or only partially open.

It could be one problem, or, in the worst-case scenario, you may be facing multiple issues that have the potential to create havoc. So, it is crucial to figure out the cause and fix it before you drive the car again.

Why Does Your Car Overheat While Idling But Not Driving?

Car overheating is alarming. When this happens while idling but not driving, it is even worse. The origin of this issue can be traced to a cooling system problem. If an issue arises, the vehicle will overheat at idle. Why?

When driving, air flow helps cool the radiator. That does not happen when idling in traffic. Common cooling system problems that cause overheating at idle but not driving include the following:

1. Low Coolant

When idling, there is slower engine coolant flow compared to when driving at higher speeds. When the coolant is low, there is not enough liquid in the system to properly absorb and transport the heat produced by the engine during idle.

Compared to when driving, the water pump is able to move coolant through the system faster to collect and dissipate heat more effectively, preventing overheating. The airflow also helps. The signs of low coolant include the following.

• Coolant level is below the minimum mark in the reservoir.
• Presence of air bubbles in reservoir or radiator.
• The engine temperature gauge reading is higher than normal during idle.

Solution:

• Fill the coolant back to the proper level if it is low. Fill it up to the “maximum” mark.
• Inspect for coolant leaks like wet coolant lines and drops of blue or yellow water under the car.
• If you find any leaks, fix them or replace the broken parts.

2. Faulty Coolant Temperature Sensor:

The coolant temperature sensor monitors the engine coolant temperature and sends a signal to the car’s computer. When faulty, it may give inaccurate coolant temperature readings to the computer.

When idling, the engine relies more on the thermostat and cooling fans to regulate temperature. A faulty sensor could cause the fans/thermostat not to turn on as needed.

However, when driving, airflow over the radiator helps cool the engine even if the fans don’t turn on. So overheating may not occur.

Symptoms of a bad sensor include:

• The temperature gauge is not working properly.
• Fans/thermostat not activating at the correct temperature.
• The engine overheated unexpectedly.
• Higher idle speed and poor starting behavior
• Check the engine light on.

Solution:

• Use an OBD-II Scanner to read for faulty coolant temperature sensor codes. Some of these include P0115, P0116, P0117, P0118, and P0119.
• If you get these codes or the symptoms, as mentioned earlier, replace the sensor. To diagnose and replace the sensor, you would get help from a professional auto mechanic.

3. Radiator Cooling Fan Issues:

When the engine is idling, there is less air flowing through the radiator for cooling compared to when driving at higher speeds. The engine cooling fan helps push more air through the radiator core to help dissipate heat from the coolant.

If the fan is not working properly, it can cause the engine to overheat when idling. Some potential issues with the engine cooling fan include:

• The fan is not turning on at all: This is usually caused by a faulty cooling fan relay, blown fuse, or issue with the wiring between the temperature switch and fan motor.
• The fan is turning on at the wrong temperature: A faulty coolant temperature sensor or switch could cause the fan to kick in too late, not giving the radiator enough cooling effect before temperatures get too high.
• The fan is turning on and off intermittently: A loose connection, faulty ground, or deteriorating fan motor could make the fan randomly turn off during idling. This disrupts consistent airflow through the radiator core.

You can tell your radiator fan isn’t working if the check engine light comes on. You can also scan for error codes. The most common are P0480, P0481, P0482, P0483, P0484, and P0485.

You may also experience AC issues. Your car might not get as cold as it used to get, and the air conditioning improves at cruising speeds.

Solution:

• Check all fuses related to the fan motor circuit and replace any blown fuses.
• Inspect the relay and wiring harness for breaks or loose connections that could disrupt power to the fan motor.
• Test the coolant temperature sensor to determine if it is accurately reading coolant temp. Replace if necessary.
• Check the fan motor operation and replace it if it doesn’t turn on consistently or at the right speed.

4. Thermostat Stuck Closed Or Only Partially Open

A thermostat regulates the flow of coolant through the radiator. When closed, it forces coolant to bypass the radiator and circulate internally to help the engine warm up faster. Once fully open, it allows coolant to flow to the radiator to be cooled.

If a thermostat gets stuck closed, it prevents any coolant from flowing through the radiator. When partially open, the thermostat will limit coolant flow and make it harder for the coolant to dissipate heat.

The symptoms of a faulty thermostat are an engine that does not reach or maintain its normal operating temperature or overheating issues like the one described during idling. The cooling fan might also run nonstop.

Solution:

• Test the thermostat using a temperature gun.
• Start the engine and rev it up till the engine reaches operating temperature.
• Measure the temperature of the engine housing and that of the radiation hose. You should get something close.
• If the hose is colder than the engine, the thermostat needs to be replaced.

Can a car overheat by idling too long?

Yes, a car can overheat from idling too long. When your engine idles, it expends a significant amount of heat to the cooling system, but the low engine RPM during idle reduces the effectiveness of the cooling system.

This issue becomes more pronounced in high ambient temperatures, especially in places like Phoenix during mid-summer, where temperatures can soar above 100°F. Being stuck in a traffic jam exacerbates the problem. The imbalance is typically resolved when the engine’s RPM increases, allowing coolant to flow through the radiator to pull excess heat from the system.

However, if you are using the engine for tasks like powering a generator, avoid prolonged idling. Maintaining an RPM of at least 650 ensures proper cooling system function. Monitoring the temperature gauge is crucial to preventing overheating, indicating when the engine has returned to its normal operating range.

FAQs:

Here are some additional queries people also frequently ask about cars overheating while idling but not driving.

Q: What should I do if my car overheats while idling?

Pull over to a safe location and turn off the engine to give it time to cool down. Do not attempt to open the radiator cap while the engine is hot, as this could cause scalding steam to erupt.

Q: How much does it cost to fix a car that is overheating?

The cost of fixing a car that is overheating will vary depending on the cause of the problem. However, it is generally a relatively inexpensive repair.

Q: How often should I check my car’s coolant levels?

It is recommended to check your car’s coolant levels at least once a month or more frequently if you drive in hot weather or take long trips. You should also check the coolant levels before each long drive.

Conclusion

If your car is overheating only when idling and not during driving, it indicates an issue with the cooling system’s ability to dissipate heat effectively at low speeds. Narrowing down whether it is low coolant, a faulty temperature sensor, problems with the radiator fans, or a stuck thermostat will allow you to identify the root cause.

Fixing cooling system defects like refilling coolant levels or replacing malfunctioning parts helps ensure optimal heat transfer occurs even without high airflow from driving. Correcting the underlying problem prevents overheating and keeps the engine functioning properly both when idling and in motion.