GMC 1994 Yukon Owner's Manual page 76

In very cold weather, 0°F
(-
18OC) or colder, the engine coolant heater can
help. You'll get easier starting and better fuel economy during engine
warm-up. Usually, the coolant heater should be plugged in a minimum of
four hours prior to starting your vehicle.
To use the coolant heater:
1.
Turn off the engine.
2. Open the hood and unwrap the electrical cord.
3. Plug it into a normal, grounded 110-volt outlet.
NOTICE:
After you've used the coolant heater, be sure to store the cord as
it was before to keep it away from moving engine parts. If you
don't, it could be damaged.
How long should you keep the coolant heater plugged in? The answer
depends on the weather, the kind of oil you have, and some other things.
Instead of trying to list everything here, we ask that you contact a GM
dealer in the area where you'll be parking your vehicle. The dealer can give
you the best advice for that particular area.
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