GMC 1995 Safari Owner's Manual page 158

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Run your 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. You will need a well-charged battery to restart the
vehicle, and possibly for signaling later on with your headlights. Let the
heater run for awhile.
Then, shut the engine off and close the window almost all the way to
preserve the heat. Start the engine again and repeat this only when you feel
really uncomfortable from the cold. But do it as little as possible. Preserve
the fuel as long as you can. To help keep warm, you can get out of the
vehicle and do some fairly vigorous exercises every half hour or so until
help comes.
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