Does Starting Or Idling Your Engine Consume More Fuel? Here's What The Data Says

Idling a vehicle after cold-starting was once a useful technique in carbureted engines. Unless you were looking to stall the engine after sticking it in gear, letting it warm up while idling was a common — and helpful — practice. However, the advent of fuel injection and direct injection (along with more precise fuel management systems) has made that practice obsolete, and idling after starting has become an outdated maintenance myth that has been debunked many times.

A similar rule applies to the myth that idling an engine saves more fuel than turning it off — especially when the vehicle is stationary, such as at a fast-food drive-through, picking up the kids at school, or waiting at a stoplight. The Massachusetts Department of Energy Resources published a YouTube video in 2015 to debunk the myth that vehicles sip less fuel when idling than restarting.

Unless you're behind the wheel of a carbureted engine that gulps more fuel during start-up, you're wasting gas by leaving a modern, fuel-injected motor idling for as little as 30 seconds. The big picture is that American drivers are collectively wasting 3 billion gallons of fuel each year by idling a car for an average of six minutes each day.

Prolonged idling not only wastes precious gas but also pollutes the environment. In a separate study by the Argonne National Laboratory, idling for as little as 10 seconds uses more gas and emits more CO2 than restarting an engine. Since a modern vehicle tends to warm up faster when driving, you're wasting gas and not doing the environment any favors by leaving a vehicle idling.

How much fuel can you save by not idling? It depends on the vehicle, engine size, idling speed, and other factors, but the average is 0.2 to 0.5 gallons per hour (0.75 to 1.8 liters per hour) of idling. It's why newer cars have automatic start-stop systems to further save gas and cut emissions while idling or when the vehicle is stationary, and that's because the data doesn't lie. Even without modern start-stop, turning the engine off makes more sense economically and contributes to cleaner air.

This begs the question: Doesn't restarting an engine spew more emissions? The answer is yes and no. Test results by the Argonne National Laboratory show that restarting an engine produces more nitrogen oxide (NO2) and hydrocarbon (THC) emissions than idling. Then again, the emissions are significantly less than from starting a cold engine. Tests show that the catalytic converter takes time to cool down (the catalyst needs heat to operate efficiently, sometimes as much as 900 degrees to 1,600 degrees Fahrenheit), and shutting down the engine and restarting it still produces fewer harmful pollutants than starting a cold engine.

Automatic start-stop technology in cars can help save fuel and reduce emissions. According to the Society of Automotive Engineers (SAE), drivers can save between 7.27% to 26.4% in fuel by using stop-start. The variances in projected savings boil down to drive cycles, engine type, and idling percentage.

If you're concerned about prematurely wearing out engine components when using stop-start constantly, don't be. Automakers have tackled the potential issue of wear and tear in stop-start systems using specialized components, including more robust starters and AGM batteries. You won't be causing an engine or system breakdown by using automatic stop-start. However, shutting down and restarting the engine frequently can degrade the 12V battery in older vehicles without auto stop-start technology.

The trick is to give the alternator enough time to recharge the battery between each restart. That means driving longer distances before stopping or restarting the motor since the alternator charges less efficiently during idling. Regardless of whether your car has an auto start-stop system, studies from various reputable sources have proven that turning the engine off and restarting it saves more fuel than leaving it idling.