It’s easier to cool the engine when you’re driving because the air flows easily the radiator. So when it’s idle, your car has to rely on a fan to draw air through the radiator. This can be a problem if your car has a mechanically driven fan that can get worn out which fails to rotate at sufficient speed in order to draw enough air and cool the engine.
Some cars have electrically powered fan that gets activated with the help of a temperature sensor to draw air to the radiator and cool the engine. So when the temperature reaches the sensor’s limit, the fan will be activated. However, like most sensors that can be defective over time, the fan may not work when your car is idle which can lead to overheated engine.
The fan shroud helps focus the air coming from the fan to the radiator in order to cool it much better. Now if this is damaged or the fan doesn’t have this, then the air just blows all over the place which lessens the air that the radiator gets. This then makes the engine overheat because there’s less air working to dissipate the heat.
When dirt, bugs, or road grime gets stuck and accumulate in your radiator, it becomes harder for the air to flow through it. As a result, it’s harder to cool the engine especially when the vehicle is stopped and the radiator have to rely on just the fan to keep it cool which is why it overheats when idle.
If clogs outside the radiator is enough for your car to overheat, how much more when the obstruction is inside? Usually, this happen on the internal cooling tubes of the radiator where reduces the flow of the coolant. Combine that with your car in idle where only the fan trying to cool the engine and you could almost expect it to overheat every time. If you don’t want this to happen, send your vehicle to car repair right away.