What Happens if You Leave Your Car Running Overnight

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Most car owners have experienced this at least once. Maybe your battery dies and you leave the car running to recharge it, then forget and go to bed. Or perhaps you were warming the car for an evening errand that you decide not to take.
Some nights, you can get home so tired that you forget to turn off the car in your haste to collapse onto something soft. What will you find when you wake up in the morning?
At that moment, you might worry whether your car is okay, or wonder if there are any maintenance follow-up steps you should take after leaving your car running all night. According to J.D. Power, this is what will happen if you leave your car running overnight.
Dual tailpipes of a silver car
It’s always a good idea to double-check that your car is off if you’re planning to stay in for the rest of the day.

Leaving your car running overnight (probably) won’t hurt it

Cars are designed to run. With infinite fuel in a perfect vacuum, your car could idle forever. Or at least until the belts wear out.
It’s a myth that idling is better for your car and, since 1990, it’s also a car maintenance myth that stopping and starting your car will do harm. Stop, start, or idle your car as much as you see fit and it won’t wear down significantly.
However, the trouble with idling your car overnight is that no system is perfect, and vehicles were designed to be monitored while running. The biggest risk of long-term idling is running out of gas, followed by an engine malfunction that overheats the vehicle while you’re asleep in bed.

What happens when your car engine idles?

When your engine idles, the cylinders turn and the alternator runs. Power is supplied to the vehicle, but the drivetrain is disengaged and the brakes are on, so your vehicle doesn’t try to go anywhere. Idling is a driving-ready state, but it only uses a trickle of fuel.
The car burns fuel slowly. The engine continues running but at a very low speed. This requires a small amount of fuel use which can run out in the time it takes you to find the idling car the next morning.
The alternator also runs. The engine and alternator run together, even when the car is idle. This means your alternator is slowly charging your battery and circling charge through the car.
The AC might run, which burns gas. If you left the AC on, you might be surprised to know that it uses a surprising amount of power and gasoline while your vehicle idles. You can wear out your fans and run out of gas more easily if your AC was on with the car overnight.
Your car also powers the electrical systems. It plays the radio, lowers the windows, locks the doors, and provides smart-screen features while the car idles.

What can happen if your car was running overnight

So what’s the worst that could happen to your car if you leave it idling overnight?
In most cases, you will find that your car is out of gas, but otherwise unharmed. This is why it’s a good idea to keep one spare gas can in your garage for exactly this kind of face-palm-worthy car owner mistake.
However, it’s important to check for other potential, more serious issues, before you climb back into the driver’s seat.
When your car idles, it doesn’t charge the battery very fast. With the AC and radio on, you can easily drain the battery faster than you fill it. This can result in a dead car battery when you find the car in the morning.
And while a well-tuned engine will run cleanly all night long with enough fuel, if there’s a worn or malfunctioning part under the hood, a night spent idling in the garage can easily overheat your engine. An overheated engine can lead to a broken belt or fan blade.
The most serious thing that can occur is the garage filling with fumes. It’s vital to open up your garage when the car is running since gasoline engines emit carbon monoxide from the tailpipe, which is toxic to humans and pets.
To prevent this, it’s a good idea to leave your garage door cracked, especially if the car is running. But this is also why you should make sure your car isn’t idling so you know your garage or home won’t be filled with carbon monoxide when you wake up.
It is most likely safe to leave your car running overnight. In a perfect world, modern engines with full tanks of gas can run for many hours without concern.
This is why we can have fun tailgate parties, car campouts, or just sit and listen to the radio for hours without worrying about if the car is okay. But it’s always a good idea to check on the gas level, battery power, engine heat, and air quality before taking it back out on the road.

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