How Long Can You Leave Your Car Running?

Leaving a car running for a short period of time should not have any ill effects on a car. But there are many myths surrounding how long you can leave a car idling before issues may arise, especially for older vehicles. One is that a car consumes less gasoline if it’s left idling rather than just shutting it off and restarting. Another is that extended idling will damage cylinder walls—this is only true for older cars, particularly some early 2000s Porsches.

This article explains the effects of extended idling, and provides tips to avoid wasting fuel, reduce emissions, and prevent premature wear on the engine.

The Basics of Internal Combustion

In general terms, the internal combustion process on a common piston engine is:

• The air/fuel mixture ignites via the spark plug inside the combustion chamber, or in diesel engines, the compression of the air/fuel mixture ignites itself
• This reaction causes each piston to push down and continue spinning the crankshaft
• As the piston comes back up inside the cylinder, it pushes the combustion’s spent exhaust out of the cylinder via the exhaust valve (or valves) and the process repeats
• Idle speed is measured in how many revolutions per minute (RPM) the engine’s crankshaft is spinning at, which is a primary component in creating power to move the car down the road

As the car drives, the combustion process continually speeds up and slows down, depending on if the vehicle is accelerating or decelerating. It’s also under what’s referred to as load, which means the engine isn’t just being revved up while sitting in place but is actually facing resistance from the drivetrain and overall vehicle’s weight to get up to speed and propel it along.

Why Leaving the Car Running Is Not Healthy

On the whole, the healthiest way to treat an engine is to let it briefly idle after startup and then drive it gently until it’s fully up to operating temperature. This is to ensure that oil and coolant are at the right temperature, circulating around the engine with ease, and in turn, the least amount of gradual, normal wear is happening to the engine. 

If an engine is sitting at idle and not under load, it’s actually operating inefficiently in one of these ways:

• The air-to-fuel mixture is usually running rich, meaning it’s using more fuel than air to keep combustion going
• Oil and coolant aren’t circulating around the engine efficiently
• Air intake temperature is quite high, as no fresh air is getting pushed into the intake via the forward motion of the vehicle (hot air is less oxygen-dense)
• If it’s an older engine, or one with several hundred thousand miles, there’s a chance that fuel is making its way down past the pistons’ rings and mixing with the oil, thus diminishing the oil’s lubrication properties
• The car needs to rely more on accessories to stay cool, such as the fan (or fans), which shortens these components’ service intervals

Leaving a car idling for an extended period can cause premature engine wear. When a car is running, all the components create conditions where there’s more heat and less oil and coolant flow, engaging the cooling fans and electric water pumps, and wearing down the vehicle’s catalytic converter.

There are several scenarios when many drivers leave a car running for an extended period of time. One is while stuck in traffic, as the car is sitting for long periods of time in between crawling along. Another is letting it warm up during winter months—nobody enjoys driving a car before its HVAC system is providing ample heat.

However, cutting down on these instances of inefficient operation is easy. Newer cars have an auto stop/start system, which automatically shuts off the engine when it's not in use. Then the car fires it back up in a split second before carrying on, usually when the driver takes their foot off the brake. Another is simply wearing adequately warm clothing in the winter and giving the car less warm-up time while motionless.

Myths and Misconceptions About Idling the Car

Auto start/stop systems on modern engines can help reduce emissions, but they don’t always fire up the engine as quickly as needed, making quick turns or fast acceleration tricky. They also may turn the car off at times when idling would be more convenient. A shut off engine also affects the temperature of the car; if the engine isn’t running, the air conditioning’s refrigerant isn’t circulating. That means the interior’s ambient temperature may rise, which can be uncomfortable on a hot day.

There are wildly different claims out there about how much gasoline an engine burns while sitting at idle, but a study done by the US Department of Energy nearly a decade ago shows that it may not be as much as previously thought. But it is still true that idling a car for too long does use up gasoline that isn’t being used for transportation, which increases the driver’s overall transportation costs.

Summary

Remember that when cars are left running for extended periods of time, the idling causes premature wear and increases fuel consumption and emissions. Generally, the less amount of time left idling the better, and it’s more beneficial to give the engine a few seconds after startup to circulate fluids, and then drive it gently until it is fully up to temperature.

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