Left Car Idling For 5 Hours – Whar Are The Risks?

Idling is when the car is not moving, but the engine is left running at a low speed. It is a common practice among drivers. Even though it is environmentally unfriendly and even frowned upon by some state laws, idling is sometimes unavoidable. A typical idling scenario that presents itself almost every day is the rush hour traffic. This is the period of time in the day when the traffic congestion is at its peak. According to this report, commuters waste about 54 hours every year stuck in traffic. 

You start to get the whole picture when you consider the fact that most of this time will be spent idling. It is no wonder the transportation sector has the highest impact on global warming. But traffic alone isn’t when drivers idle, the habit has become so ingrained in most drivers that they prefer to idle rather than turn off the car. While the duration of idling differs from one driver to the other and from one situation to another, there are some who take it to the extreme. 

In this article, I will be talking about one of such extreme idling situations, where drivers idle for very long hours (up to 5 hours). Do you want to know what happens? Then keep reading to find out. 

What happens if I leave my car idling for 5 hours? Any risks?

A lot happens if you leave your car idling for more than 30 minutes, not to mention doing so for 5 hours. Before I dive in, let me answer the question, Yes, it is risky to idle for so long. Doing so would put your health, your car, and the environment at risk. Here is why.

Idling has long been considered harmful to the environment. To curb its impact on global warming, many countries and states made a law prohibiting idling for a specific length of time. The exact “acceptable” idling time will depend on the state law, country, and weather conditions. In Washington DC, for example, you get to idle your car for 3 minutes maximum. Anything more than that will attract a fine of $5000. 

But in Pennsylvania, drivers are permitted to idle for up to 20 minutes when the temperature is below 40⋄. Other states like Las vegas aren’t so lenient, here, idlers can be fined up to $10,000 for just 15 minutes. You can read more about idling laws in the US here. In the UK the story is no different, albeit a bit more lenient. Here are some of the reasons why idling (especially for 5 hours) is not only risky but illegal in some places:

Additional wear to the engine

One hour of idling is equivalent to 30 miles of driving, on average. The actual value varies but going by this approximation, it, therefore, means that 5 hours of idling is equivalent to 150 miles of driving. What happens to your car’s engine after driving continuously for 150 miles? Well, I’m sure you’re starting to get the picture. If not, let me give you another statistic; most car owners are recommended to perform a full servicing of their car’s engine every 12,000 miles or 12 months, while interim checks are recommended every 6,000 miles or 6 months. Now, this is an averaged value, you should check your car’s manual to see the recommended servicing times. 

Regardless of how often your manufacturer recommends you service the car, I’m trying to make a point with these stats: excessive idling will take up most of the car’s mileage. Meaning that your car has to be serviced more often than it needs. Every 5 hours of idling time is equivalent to 150 miles of driving time (lost to idling). So if you idle for 5 hours every day, that means you’ll be due for full service after 400 hours (80 days or about 3 months), during which time your engine has covered the stipulated 12,000 miles. 

In essence, your engine is wearing faster than it should. The oil filter gets dirty faster, and the head gasket, spark plugs, cylinder rings, and piston rings deteriorate faster and would need to be replaced. But that’s not all, as the engine wears down, the damage will contaminate other important components of the vehicle, like the oil.

It introduces a lot of hydrocarbon to the oil, which reduces its life

One of the downsides of idling an engine is that it leads to incomplete combustion of fuel since the engine is not operating at its optimal temperature. This leaves behind soot and other hydrocarbon by-products that eventually find their way into the oil. Aside from that, the excessive wear of engine parts (due to idling) increases metallic residues in the oil. Both factors shorten the lifespan of your engine oil, requiring you to change the oil sooner. 

Oil change intervals are usually within the range of 5,000 to 7,500 miles and up to 15,000 miles if synthetic oils are used. This mileage may seem impressive, but if you go back to our previous calculation, you’ll realize that you can easily burn through them in a matter of days if you idle excessively. Also, excessive idle times can lead to oil shortages. This will result from several reasons, such as oil burn and leakages from worn engine parts. 

Wastes gas

There is a popular myth that turning the engine off and on regularly consumes more fuel. Although this myth has long been debunked, many drivers still carry this mentality around. Here is the fact, except if you are in traffic, it is much more economical to turn off your engine than to idle it. Long idling hours coat the spark plugs with residues of incomplete combustion, making them inefficient. 

As such, you may experience a reduction in fuel economy (could be up to 30%). The Environmental Defense Fund (EDF) states that idling for one-hour burns between 1/5 to 7/10 of a gallon of fuel in cars and up to one gallon of fuel in trucks. If you are to idle for 5 hours, then you will be wasting between one to three and a half gallons of fuel in an hour. The impact of these is much more immediate, considering the current high gas prices.

In the US, the average gas price is about $3.68, while the average price of diesel is $4.84. If idle for 5 hours, then you have wasted between $20 to $60 (approximately), for gasoline engines and between $25 to $87 (approximately) for diesel engines. This is a steep price to pay for a few minutes of convenience. For more information on this topic, check out our article on how much gas you burn when idling.

Damages the catalytic converter (mostly in older cars)

The internal combustion engine was a revolutionary invention that has only improved over the years. But it still has one drawback, the hydrocarbon by-products. These gasses and particulates released at the end of the combustion process in the ICE are toxic. The catalytic converter is a device that makes driving the ICE possible without releasing a large amount of toxic and poisonous by-products into the air. 

It converts most of the toxic gasses into less harmful gasses using a catalytic reaction that takes place within its honeycombed chambers. The catalytic converter is expensive to replace and can be damaged by excessive idling. The reason is that the device operates better at a high temperature (not excessively high, as you will see later). This high temperature accelerates the catalytic reaction inside the chambers. 

But during idling, the engine is not operating at peak temperature, producing more unburnt fuel. The unburnt fuel (which also damages the spark plugs) will eventually find its way to the catalytic converter and clog it up. In addition, this unburnt fuel can ignite in the catalytic converter causing its temperature to rise higher than usual (known as backfiring). Both processes can damage the catalytic converter. Additionally, some vehicles start burning oil when idling for long periods of time.

Extra wear on the battery

You may not be aware of this, but idling puts more burden on your battery and may cause it to wear out faster than usual. When you start your vehicle, the battery automatically kicks into action providing power that brings the engine to life and also powers other electronic components. To make up for the huge power demand, the alternator keeps the battery sufficiently charged at all times. 

But when idling, the alternator operates slowly and produces much less current compared to when you are driving the vehicle. This means the battery is not getting charged fast enough and still has to power so many accessories like the AC, headlights, radio, and even charging phones. The long-term impact is that the battery deteriorates quicker and would need to be recharged or replaced. 

Harmful emissions 

Hydrocarbons and particulate matter (referred to as emissions) are by-products of the internal combustion engine. When these emissions are released into the atmosphere, they cause many issues, including health-related diseases like asthma and climate change. Not only does idling use more fuel, but it also generates about 30 million tons of Carbon dioxide yearly. Do you recall our discussion about the catalytic converter? Yes, they were designed to reduce emissions from our vehicles. But excessive idling will prevent these important components of the vehicle from functioning as they should. With that, more harmful emissions will be released into the environment.

What should you do if you left your car idling for 5 hours?

There is not a lot you can do if you forget your vehicle idling. If the car did not run out of fuel or get stolen, you could take the car for a 30 minutes drive to ensure the battery will be recharged properly and any unburnt fuel is out of the system. Taking your car for a spin will also allow you to ensure that everything is working smoothly.

If the vehicle does not start, your car might have ran out of fuel. You should add some gas and turn the key to the first position (ON) and back several times. This will eliminate any air that has gotten into the fuel lines. If the car is still not starting, you should seek a professional mechanic to inspect your fuel delivery system.

A second possibility is that your battery is now dead. You should check this with a voltmeter to see if it is still charged. You could also check your battery by turning on the radio with the engine off. If the radio works, it should tell you that the battery is fine and there might be another cause. Here are 7 possible causes that might cause your car to not start when the battery is still charged.


So what happens if you leave your vehicle idling for 5 hours? On the off chance you don’t get arrested and slammed with a huge fine, you would damage your car and contribute negatively to global warming. Also, the financial impact of your actions will accrue over time in the form of regular maintenance, oil change, replacing the catalytic converter, battery, and all other components that are affected by idling. Is it worth it? No. There is no benefit to idling, and although it is unavoidable (sometimes), you should consciously try to avoid idling your engine when you can.