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Kate Hayes still recalls the advice her father gave her years ago before she'd take her Pontiac Sunfire for a drive on brutally cold days.

"He told me to let it warm up," the 31-year-old Burnsville resident said in a recent phone interview. "But I've always kind of wondered. Should you let your car run on cold mornings before driving it?"

With Minnesota in the throes of winter, Hayes posed the question to Curious Minnesota, our community-driven reporting project fueled by readers.

It's totally unnecessary, even on the most frigid days, said Douglas Longman, manager of engine research with the U.S. Department of Energy's Argonne National Laboratory. Modern cars have electronic fuel injection, so drivers need to let their vehicles run for only up to 30 seconds before pulling out of the driveway. Motoring allows the engine to warm up faster and reduces wear on engine components, he said.

"Prior to fuel injection, there was a reason to let a car warm up," Longman said. "It just didn't run well. With the electronics today, that reason has gone away."

Not everybody apparently got the memo. A 2009 study from Vanderbilt University surveying 1,300 motorists found the average respondent believed a vehicle should be warmed for five minutes before driving in subfreezing conditions.

The same study found that the average respondent believed it is better to idle for over 3½ minutes than to turn a vehicle off and restart it when it is time to move.

Fuel economy declines by 10 to 20% in city driving during cold weather, and warming up the vehicle adds to the loss, according to the U.S. Department of Energy and the Environmental Protection Agency. Idling motorists, not including when sitting in traffic, waste about 3 billion gallons of fuel and generate around 30 million tons of carbon dioxide annually, the Energy Department said.

Letting a car run won't get you anywhere, but it could get you a ticket. In Minneapolis, it's illegal to let a car idle for more than three minutes in a one-hour period, or for more than 15 consecutive minutes in any one-hour period when temperatures are at zero or below.

Leaving a running car unattended can also be a golden invitation for thieves. Cars with keys or key fobs left inside could mean instant transportation for the unscrupulous. Last year, 3,904 vehicles were stolen in Minneapolis, according to police department data. More than 1,250 of those were running unattended when swiped.

Hayes' dad was likely relying on practices from 30 and 40 years ago when it was almost a necessity to warm a vehicle before hitting the road. Before the late 1980s and early 1990s, cars relied on a carburetor to provide the engine with the right mix of fuel and air. Carburetors didn't work well until the engine warmed up, and a cold engine would make a vehicle run rough or stall.

"You really could not drive on a cold engine," said Joe Wiesenfelder, executive editor of Cars.com. "Modern cars are made to run after 30 seconds. The secret is to not drive too aggressively until the engine is fully warmed up. In the winter, it takes a bit longer to reach that point."

Of course, it's also incumbent on keeping a vehicle properly maintained, including getting regular oil changes and having clean oil filters, he said.

The rules are slightly different for electric vehicles, Wiesenfelder said. Warming them while they are plugged in will extend range by keeping the battery at full capacity. Using the battery to warm the cabin while driving can reduce range, he said.

Perhaps the biggest reason motorists run their vehicles is because they don't like getting in a cold car.

"People like to justify their comfort," Longman said. "They say their car needs it."

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