Why is it bad to let a car idle on cold mornings?

Dear Car Talk

Dear Car Talk | May 01, 2003

Dear Tom and Ray:

I know we aren't supposed to let our vehicles sit outside on frigid mornings, motor on, idling to "warm up" while we quaff another cup of Earl Grey. I've heard you mention that before. What I can't remember is why. When I quoted this rule to a co-worker, she sniffed "But that's counterintuitive." Can you tell me why it's bad? -- Cathy

TOM: Well, these days it's only bad because it's wasteful.

RAY: It used to be bad for the car. Twenty years ago, when most cars had carburetors, fuel would pour into the cylinders on a cold start. And left to run in cold-start mode for 15 or 20 minutes, all that excess gasoline would dilute the oil, which will shorten the life of your engine -- not to mention your catalytic converter.

TOM: Fortunately, all newer cars are fuel-injected now, and fuel delivery is monitored by computer. So gasoline will no longer just pour into the cylinders, because the computer determines the precise amount of fuel to send in through the injectors. So, you will not do any harm to your car these days by warming it up -- even for a long time.

RAY: It is, however, still completely unnecessary to let your car sit and warm up. Engines warm up fastest when they're driven. So, as long as the engine starts and keeps running when you put it in gear, you're good to go. Just drive it gently for the first few minutes (don't jump right on the highway and go 65), and you'll warm it up quickly and efficiently.

TOM: By letting it run, you're just wasting gasoline and polluting the neighborhood.

RAY: But, you might ask, isn't it worth a little pollution and wasted natural resources so my feet can be toasty the moment I get in my car? Well, that's a personal, ethical decision that each one of us has to make by ourselves -- like whether to leave the water running while we brush our teeth.

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