GMC Envoy Owner's Manual page 351

CAUTION:
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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. Do not top off or overfill the
tank and wait a few seconds after you have finished
pumping before removing the nozzle. Clean fuel
from painted surfaces as soon as possible. See
Washing Your Vehicle on page 5-89.
When replacing the fuel cap, turn it to the right
(clockwise) until it clicks. Make sure the cap is fully
installed. 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 3-42.