GMC 2007 Acadia Owner's Manual page 359

You can run the engine to keep warm, but be
careful.
CAUTION:
{
Snow can trap exhaust gases under your
vehicle. This can cause deadly CO (carbon
monoxide) gas to get inside. CO could
overcome you and kill you. You cannot see
it or smell it, so you might not know it is
in your vehicle. Clear away snow from
around the base of your vehicle, especially
any that is blocking the exhaust pipe.
And check around again from time to time
to be sure snow does not collect there.
Open a window just a little on the side of
the vehicle that is away from the wind.
This will help keep CO out.
Run the engine only as long as you must.
This saves fuel. When you run the engine, make it
go a little faster than just idle. That is, push the
accelerator slightly. This uses less fuel for the heat
that you get and it keeps the battery charged.
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