GMC 2004 Sierra 1500 Pickup Owner's Manual page 408

To remove the fuel cap, turn it slowly to the left
(counterclockwise). It will require more effort to turn the
fuel cap on the last turn as you loosen it.
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CAUTION:
If you spill fuel and then something ignites it,
you could be badly burned. Fuel can spray out
on you if you open the fuel cap too quickly. This
spray can happen if your tank is nearly full, and
is more likely in hot weather. Open the fuel cap
slowly and wait for any "hiss" noise to stop.
Then unscrew the cap all the way.
5-8
Be careful not to spill fuel. Don't top off or overfill your
tank and wait a few seconds after you've finished
pumping before you remove the nozzle. Clean any
spilled fuel from painted surfaces as soon as possible.
See Cleaning the Outside of Your Vehicle on
page 5-120 .
When you put the fuel cap back on, turn it to the right
(clockwise) until you hear a clicking sound. It will require
more effort to turn the fuel cap on the last turn as you
tighten it. Make sure you fully install the cap. The
diagnostic system can determine if the fuel cap has
been left off or improperly installed. this would allow fuel
to evaporate into the atmosphere. See Malfunction
Indicator Lamp on page 3-49 .