If your car overheats when idling, you probably have an issue with the coolant system.
There are numerous reasons why this can happen and this article will show you some of the most common ones.
Besides this article, you can find another one devoted to engine overheating in general, so if you want to read it click here.
1. LOW COOLANT LEVEL
Before inspecting anything else, check the coolant level.
See that the level is between minimum and maximum. If it’s under minimum, open the coolant bottle and add some.
On the other hand, if it’s over maximum, get the excess coolant out.
The car overheats when idling both when there’s too little or too much coolant.
2. AIR IN THE COOLANT SYSTEM
This is a more rare reason and you should consider it if there were some kind of recent repairs done on the coolant system.
Sometimes there might be a loose clamp, loose screw, perhaps a punctured hose, or else which lets air into the system.
One more common reason is a loose cap on the coolant bottle. This usually happens when the cap is forgotten to be tightened.
When the air gets into the system, it causes the car to overheat when idling because of extra pressure within the system, much like an air bubble.
3. LEAKS IN THE COOLANT SYSTEM
Leaks cause the system to lose coolant and this will inevitably lead to overheating.
The first spots you should check are punctured or damaged hoses, loose or broken clamps, various seals, connectors, and else.
Overheating will especially be noticeable when idling in higher outside temperatures when the system is under more stress.
In a less serious case, you’ll recognize the lack of coolant as the temperature gauge will stand a bit over the middle, more towards the red zone.
The best way to spot this problem on time is to regularly check the coolant level and take a short glimpse under the car.
Also, a brief check around the engine bay from time to time is recommendable.
If you see a pond of coolant under the car, then you know the reason why the car overheats when idling.
For more information on how to find a coolant leak, click here to read a separate article on that topic.
4. LOOSE OR WORN SERPENTINE BELT (FAN BELT)
Check this out, especially if your car has a belt-driven fan (opposed to an electric one).
Especially check this out if you hear a squeaking or weak screeching sound coming from the engine bay.
This is the first sign that the serpentine belt, also known as a fan belt is worn out and on its last legs.
When checking, take a look at the upper side of the belt but more importantly try to twist a bit with your hand and see the lower side. The one that actually touches the pulleys.
Check for cracks or tears. If they are present, you may have found you’re the problem. A worn-out fan belt will slip under strain causing the fan to work with less efficiency.
All this means less cooling of the whole system and in the end, even overheating.
This is why changing the belts on your car is very important. If you want to read more about serpentine belt problems which are similar to what we’ve mentioned, click here to read the article.
5. BAD THERMOSTAT
The thermostat functions as a valve inside the coolant system. It regulates the coolant flow depending on the engine and coolant temperature.
When the engine is cold, it’s closed and there’s no circulation enabling the engine to reach working temperature much faster.
Once it gets hotter the thermostat opens enabling full circulation.
When the thermostat gets stuck in the open position, the engine can’t gain a working temperature.
But worse, if it’s stuck in the closed position, there is no proper circulation and the overheating starts,
Not only when idling but also during driving.
It’s especially noticeable when idling since the car is at standstill and theirs no airflow through the radiator.
With this problem, the overheating starts almost immediately after the engine has gained working temperature.
To paint a better picture: the temperature gauge slowly climbs to the middle and then swiftly goes into the red zone, usually in a matter of minutes or even seconds.
6. BAD THERMOSWITCH
The thermo switch engages the electric fan located on the radiator once the coolant system reaches a certain temperature.
Thermo switches are electric switches that have a bimetallic component that is sensitive to temperature. Material fatigue is one of the main reasons why this switch stops working.
It’s very important when the car is idling. If it doesn’t turn on the fan in time, the car will quickly overheat since there’s no natural airflow.
One of the most obvious symptoms of this problem is that the radiator fan doesn’t start working at all despite the rapidly climbing engine temperature.
7. CLOGGED RADIATOR OR OTHER CLOGS IN THE COOLANT SYSTEM
A clogged radiator is a reason why the car overheats when idling that mostly appears in older cars.
Over time, due to material fatigue, filth or scale build-up within the radiator, or perhaps physical damage, the coolant flow becomes weaker.
When it does, the car will start to overheat. Less when driving, more when the car idles.
In this case, you’ll notice that the engine temperature is always a bit higher than it should be with a tendentious to climb even higher when idling.
So, the engine is always in a state of mild overheating, so to say. Also, there might be a constant pressure build-up inside the coolant bottle.
This problem can happen even with newer cars: for instance when if there was some kind of physical damage to the radiator.
Clogs can also appear within other parts of the cooling system, like within hoses of a smaller diameter, within the engine block canals, inside the heater core, or else.
8. ELECTRIC RADIATOR FAN PROBLEM
The radiator fan, which is located near or on the radiator, has the task of cooling the system when there’s no natural airflow.
It’s crucial for cooling while the car is idling or for instance in a long uphill drive when the system is under extra strain.
You’ll notice this problem easily. There will be an absence of the well-known working sound of the fan when the car idles or when you turn off the heated-up engine.
Instead of hearing the strong sound of a ventilator, there will be nothing.
With this symptom, have in mind that it may not be just the ventilator but other parts of the coolant system like the mentioned thermostat or thermo switch.
Another possibility is that the fan may work, but not properly. In this case, you may hear only a weak ventilation sound or a clunky, scraping, or grinding sound.
When this happens, you should check the electric fan motor itself (for instance the bearings, bushings, or else).
9. WATER PUMP PROBLEM
This is one more overheating problem that appears for two main reasons: material fatigue and lack of proper maintenance.
The water pump is the main factor for coolant circulation within the engine. It’s under constant load and strain and if not checked and replaced on time, it will cause overheating when idling and when driving.
The most common reasons for problems are the pump bearings, the impeller, and seals.
Water pump problems are best noticed as a weak overheating at the beginning that gradually becomes a full-blown problem.
It can be accompanied by leaks on the water pump as well as frequent loss of coolant.
These kinds of problems are basically rare or nonexistent if you replace the pump on time. On most modern engines they are driven by the timing belt and are replaced at the same time.
So, if you do regular maintenance and replace the belt and pump on time, there should be no worry.
10. PROBLEMS WITH THE AC COMPRESSOR, STEERING WHEEL PUMP OR ELSE
As you probably know, the AC compressor, the steering wheel pump, the alternator, or other parts of the auxiliary systems on your car are belt-driven.
They’re all connected to the engine via the serpentine belt.
If they’re not working properly, they put extra load on the engine and this is most evident when the car is idling.
When idling, the engine has the least power but the most load and strain if everything is turned on (climate control, headlights, or else).
The most common problems from these appliances come from worn-out bearings and pulleys as these take most of the load.
Tell-tale warning signs for this problem are squeaky or screechy sounds coming from the engine bay caused by a strained serpentine belt.
This may also be accompanied by slight overheating because of added load to the engine.
So, if you see that everything is OK with the coolant system, make sure to consider this reason.
Reasons for why a car overheats when idling, as you’ve seen, are numerous.
It may be just a minor fault or a serious, more costly problem. Whatever the case, the most important thing to do is check out the problem as soon as possible.
Overheating problems are notorious for starting as a small problem and becoming a big one in no time. For instance, a small, barely visible leak can become a flood in a matter of minutes.
Also, every overheating affects the engine and reduces its lifespan. In severe cases, it can even lead to a partial or complete engine overhaul.
So, the moment you see the temperature gauge needle going over the middle, take action, try to find the reason, and solve it.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com