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

idling, winter
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

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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idling, winter

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