It’s strange when the engine drives perfectly fine, with no overheating or warning signs on the dashboard. Then as soon as you stop at traffic lights, the temperature gauge shoots up, and the dashboard warnings lights start blinking at you. Sound familiar?
Having a car that only overheats when idling is a lot more common than you think, and 99% of the time, the cooling fan failing to operate is the cause. The problem with assuming it’s the cooling fan is a series of events inside the engine that happen before it decides I’m too hot; now, start the cooling fan. Anyone of them can cause the same issue and even impede what the cooling fan should do if it works!
Why is my car only overheating at idle?
A car overheating can sometimes only be evident when idling because when driving, even at low speeds, say 10 – 15mph, air passes through the radiator, cooling the coolant. Providing the coolant is moving around the engine correctly, and nothing obstructs the coolant flow, the temperatures are then under control. Even when there is a fault with the cooling fan or thermostat and the flow of coolant is impeded, just the cool air getting in and the hot air being forced out of the engine bay provides some element of cooling.
However, when stationary at idle, there is no moving air. This is why cars have a cooling fan to generate the required airflow through the radiator, and 90% of the time, the engine overheating at idle is a cooling fan-related issue. There is a chance it’s not a cooling fan-related issue, and it’s one of the other items, like a faulty thermostat or water pump indicated below.
6 causes of a car that’s overheating when idling
Below are the six most common causes of a car that overheats when stationary with the engine running.
1. Cooling fan issue
A cooling fan issue is the most common cause of a car that overheats, but only when idling. This is because its job is to provide air to cool the hot coolant passing through the radiator when there is not enough air because you’re stationary idling, and then remove that hot air from the engine bay. There are different types of radiator cooling fans, that both have their separate issues. One is mechanically driven the other is electrically driven.
Engine driven fan
A mechanical or viscus cooling fan is attached to the pulley spun by the serpentine belt. The engine’s speed determines the speed and, thus, the cooling capacity or airflow of the fan. On a mechanical fan, the cooling fan is never off and always spins where, as a viscous cooling fan has oil and a clutch system that slips until the engine gets to a required temperature where it engages and turns at full force; either way, they are both engine driven.
A few issues can happen to an engine driven to stop it from performing. One is the blades can snap, usually caused by impact from debris. Another is the pulley bearing or tensioner can fail, and the pulley fails to spin, or the serpentine belt can break, which means nothing is turning the fan. Lastly, with a viscus fan, specifically, the clutch mechanism can fail or seize so that the fan will stay consistently disengaged.
Electric cooling fan
An electronic cooling fan is slightly different in that it can fail for so many reasons.
- Broken blade
- Failed fan motor internals
- Faulty wiring to the fan
- Faulty fan switch
- Faulty fan relay
- Blown relay
Lastly, although not directly the fan is causing the problem with a faulty coolant temperature sensor, the electric fan will not kick in when the coolant hits the fan switch on temperature. Or the fan can continuously run on when not needed.
Regardless of the cause of the electric fan failure, the engine will overheat without the operation of a cooling fan when idling.
2. Water pump failure
The water pump is responsible for circulating the coolant from the engine to the radiator to be cooled. If the water pump internals fail, an inadequate amount of coolant will be pumped around the machine. One specific component in the water pump prone to failure is the impeller. Over time, the impeller can wear out because of contaminated coolant or become damaged from running low on coolant.
At idle, a lack of proper coolant circulation can result in overheating. This might not happen when driving because, at higher engine speeds, when driving, the water pump can generate enough pressure to compensate for the reduced flow.
3. Faulty thermostat
When the thermostat functions correctly and reaches operating temperatures, it opens and allows coolant to flow from the engine to the radiator. However, suppose the thermostat is stuck closed or, in this instance, more commonly partially closed. In that case, it restricts the flow of coolant to the radiator, impeding the car’s ability to cool itself via the coolant.
When the vehicle is moving, there is enough airflow through the radiator to compensate for the reduced coolant flow. However, when the engine is idling, there is less airflow, and the restricted coolant flow caused by a faulty thermostat will result in the engine overheating.
4. Low coolant level
Coolant, also known as antifreeze, absorbs and dissipates engine heat. It is circulated through the engine and radiator, regulating engine temperatures. If the coolant level is low, there isn’t enough coolant to absorb and carry the heat to the radiator.
This is more obvious when the engine is idling because the reduced flow rate, because the level is low alongside the reduced airflow through the radiator, means the engine can only dissipate so much heat, which may not be enough to keep the car from overheating. However, when driving at higher speeds, even with a low coolant level, there may be enough coolant moving quickly which is keeping the vehicle from overheating. The other problem with a low coolant level is that it’s probably not just a low-level but something causing a coolant leak.
5. Faulty temperature reading
One thing to consider is are the correct temperatures being displayed on the temperature gauge. Aftermarket or retrofitted temperature gauges are common for failing; this can lead to you thinking the car is overheating when idling, and you may not be.
Another item that can fail is a coolant temperature sensor, which provides the ECU and the gauge on the dashboard with the reading. A bad coolant temperature sensor can provide sporadic readings. I remember an old Ford Focus of mine. The coolant temperature sensor failed, and a few seconds after idling, the engine was overheating, wouldn’t rev, and entered limp home mode. Yet the engine wasn’t actually overheating.
6. Blocked radiator core
The radiator relies on air passing through the fins of the core, which cools and dissipates the heat from the tubes carrying coolant. The radiator core can get blocked by leaves, dirt, debris, and rust build-up, obstructing airflow through the radiator.
This is most noticeable when the car is idling because the cooling fan is trying to suck air through the core but cannot if the core is blocked, so the engine overheats. When driving, the air still hits the front of the radiator core, providing some level of cooling.
Symptoms of a car overheating when idling
One thing to be sure of when a car is overheating is not to rely on the gauge’s temperature reading; for one, that could be faulty, or a coolant temperature sensor could be faulty. In this case, the reading will not be correct. Although if your car is overheating, do not take any chances but just be aware of the other symptoms, such as:
- Temperature gauge reading too high
- Warning lights on the dashboard
- Steam coming from the engine bay
- Coolant leaking out of the radiator coolant cap
- Noisier engine
If you notice any of the above, it is essential to switch off the engine immediately. You could cause much more damage to components that haven’t necessarily failed yet, like the head gasket, which can be expensive to put right.
Diagnosing the fault
To correctly diagnose which component is causing the engine to overheat, you need to check the operation of each component. This is the order and how I would check for this type of fault:
- With the engine of and cold, check and adjust the coolant level.
- Check no debris or anything is blocking the radiator core.
- Check under the engine bay, ensure the cooling fan is complete with no blades missing and the serpentine belt is still present and on the pulleys correctly.
- Start the engine and allow it to get up to temperature.
- Keep an eye on the temperature gauge; there should be a sudden small temperature drop indicating the thermostat has opened.
- Alternatively, feeling the temperature of the top and bottom coolant hoses can indicate if coolant is flowing into and returning out of the engine. Be careful not to burn yourself are get your hand caught in any rotating components.
- Listen or look at the cooling fan. It should now be spinning at full speed.
- Lastly, look at the coolant reservoir to indicate a good flow of coolant returning to the reservoir.
If you cannot identify a fault from these checks, you may need to get the car to a mechanic to inspect it or plug it into a diagnostic machine to check for any coolant sensor-related faults.
Is it safe to drive if its only overheating when idling?
To give you some false hope, yes, it’s OK to drive when the car is only overheating when idling, BUT how will you know that you won’t be faced with a traffic jam or have to idle for precisely long times at traffic lights? You can’t predict these things when driving, so avoid it.
Another significant factor is, unless you know the exact cause of the fault, be aware that the car could start to overheat when driving at any point. So if you need to limp the car home or to a place of repair, pay attention to the temperature gauge, how the car drives, and any warning lights. Don’t hesitate to pull over and let the car cool sufficiently before continuing the journey.
Although the most common cause of a car overheating when idling is a cooling fan not operating, the problem could lie with one of the other issues in this article. However, if it is the cooling fan now, the question comes should you continue driving? Well I recommend you a read this article next, can I drive the car if the radiator fan is now working?