Chevrolet 2008 Avalanche Owner's Manual page 370

To remove the fuel cap, turn it slowly counterclockwise.
It will require more effort to turn the fuel cap on the
last turn as you loosen it.
CAUTION:
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Fuel can spray out on you if you open the
fuel cap too quickly. If you spill fuel and then
something ignites it, you could be badly burned.
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.
5-10
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 5-102.
When replacing the fuel cap, insert the tether in its hole
before tightening the cap. Turn the fuel cap clockwise
until it clicks. It will require more effort to turn the fuel cap
on the last turn as you tighten it. 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 3-43.
The TIGHTEN GAS CAP message will be displayed
on the Driver Information Center (DIC) if the fuel cap
is not properly installed. See DIC Warnings and
Messages on page 3-59 for more information.