Internal combustion engines work by exploding diesel or gasoline and harnessing the power of the explosion to drive a piston, a crankshaft, and finally, your wheels.
These explosions generate a very problematic by-product – heat. A great deal of a car’s engine design is focused on cooling the engine block and preventing it from warping, expanding beyond specifications, or even melting.
A good cooling system will keep your car within tolerable operating temperatures when cruising down the highway or when idling in a traffic jam. You shouldn’t worry about your car overheating if:
- You have enough coolant
- Your radiator is working fine
- Your vehicle isn’t overloaded
- The engine isn’t tuned beyond stock without upgrading the cooling system
How Does a Vehicle’s Cooling System Work?
Your car’s cooling system consists of:
The Coolant
Coolant is a specialty fluid that transports heat from the engine block into the radiator for heat exchange. While water can do the job, modern vehicles use special coolant fluids to make the process more efficient.
- As the coolant flows through the engine block, it heats up because the engine block is hot
- The coolant is pumped back to the radiator, where an incoming stream of air cools it down to a lower temperature
- The water pump pushes the water back into the engine block, and the process is repeated
A Radiator
The radiator sits at the front of the car. Its task is to radiate heat from the engine out to the incoming air stream, hence transferring all that excess heat from the engine.
Plumbing
The plumbing is an assortment of tubes and channels in the engine block through which the coolant flows to facilitate the heat exchange.
A Radiator Fan
While the radiator gets all the airflow, it needs to cool your coolant when cruising down the road. You won’t get enough flow when idling at a red light or in slow traffic jams.
This is where the radiator fan comes in. It kicks in and forces air through the radiator to keep the cooling process working optimally.
A Water Pump
The water pump pushes the coolant through the entire cooling system. It provides the necessary pressure to circulate the fluid between the radiator and the engine block.
Why Does a Car Overheat When Stopped?
A couple of things can make your car overheat as you idle. Having seen how the system works, we can quickly identify the most common culprits: The radiator fan and the coolant.
A Damaged Radiator Fan
The radiator creates the constant airflow needed to bring cool air to the radiator and keep it working efficiently. While this isn’t necessary when cruising down the highway since your motion will provide the airflow, it is crucial when the car is not moving.
That is why most modern vehicles use a thermostat to control an electric fan that will only kick in when the coolant temperature creeps up.
This ‘tells’ the system the current airflow is not sufficient to keep the coolant temperatures low enough hence activating the fan to augment it.
There are two types of fans.
An Electric Fan
The electric fan only kicks on when necessary. This improves fuel efficiency and power output since the fan doesn’t draw unnecessary energy from the engine by running even when it’s not needed.
Common failure points in electric fans include:
- A blown fuse
- Damaged wiring
- Faulty control module
- A blown fan motor
Electric fans have the advantage of running at different speeds depending on how much airflow the control module feels is sufficient to keep temperatures low without using a lot of power.
Mechanical Radiator Fans
Mechanical radiator fans are hooked to the engine mechanically using a fan belt.
This means that the fan belt is constantly running regardless of the engine speed or coolant temperature.
This adds a constant drag to the engine and slightly impacts the engine’s performance and power consumption.
Some common power failure points include:
- A damaged radiator fan belt
- A seized up fan
- Broken Fan Blades or a Missing Fan
Regardless of what fan you are using, broken fan blades or a completely missing fan is bad news. You will have nothing to move the air when parked hence your car will overheat.
This is common after an accident, vandalism, or in a very old car.
How to Tell if My Fan is the Problem
Proving your fan is the problem is easy. To start with, manually inspect the fan and ensure that:
- It is present
- It has no broken blades
- After this, you have to check if it is moving or not.
A mechanical fan can be identified by seeing a belt connecting it to a pulley on the engine block. If you see the pulley with no belt, then you are missing the fan belt.
If the belt is there, turn on the engine and see if the belt spins with the engine. If it doesn’t, perhaps the belt is damaged, or the fan is seized up. You can start with replacing the belt before thinking of replacing the fan.
ProTip: If the belt looks very old, change it altogether. It might be slipping and moving the fan slower than you should.
Does the Car Cool Down When You Start Moving
Another sure way to pinpoint the problem with your fan is if your temperatures start dropping once you start moving.
If this happens, the radiator benefits from the oncoming air when cruising, and the fan is the problem.
Low Coolant Levels or Fouled Coolant
Low coolant levels could also make your cooling system less efficient. The dropped efficiency could manifest when idling since there is no extra 60 MPH airflow to compensate.
The same could happen if your coolant is contaminated with oil due to a blown head gasket, it is dirty because it is very old, or you are using the wrong coolant.
We did a post on coolant levels. Check it out to learn more about how low coolant levels ruin your cooling system’s throughput.
A Partially Blocked or Fully Clogged Radiator
A radiator is most efficient when clean. Over time, dust and bugs block the air channels between its fins, making it less efficient. The radiator fan might not be moving enough CFM to keep the radiator working within spec.
In most cases, this could also create a problem when cruising on a hot day or towing something heavy uphill.
A quick wash of the radiator should eliminate extra gunk, bugs, and dust, refreshing your radiator’s cooling ability. If most of the radiator fins are bent, you will have to replace the entire unit for better performance.
Washing can help if you offroad and wade your vehicle in mud often or accidentally drove it into deep mud once.
ProTip: Washing a radiator is rarely necessary since all that airflow blows gunk and dirt off. If you have to, use a garden hose as a pressure washer can easily bend radiator fins. If you use a pressure washer, ensure that the nozzle is at least 3 feet away.
Other things that make the engine overheat could also cause it to overheat when idling – assuming the first thing you do after turning on your engine is idling it for a while. These range from:
A thermostat that is stuck shut.
The thermostat controls when the coolant starts flowing around the entire system. It remains shut after turning on the engine and only opens once the coolant in the engine block starts warming up.
- It will open gradually and eventually get fully open when all the coolant is up to spec.
- If it is stuck shut, all your engine will have is the coolant in the engine block. Your car will overheat fast.
- You can identify this problem by checking for coolant flow back to the expansion bottle.
- There will be no flow, and the coolant there will be perfectly cold.
Other causes include:
- Blown head gasket
- Using the wrong coolant
- Overworking your engine by overloading
- Driving in the desert or very hot environment
- Engine upgrades like adding turbos/superchargers/nitrous without modifying the cooling system
Ensure that the Car Only Overheats When Idling
Be keen and only confirm the car overheats when idling to narrow down your troubleshooting.
If you can’t risk driving a distance to prove the theory, you could simulate external airflow using some big fans to create the airflow.
Luckily, this won’t be necessary if your physical inspection shows that your coolant levels are low or your fans are not moving at all.