Chevrolet Silverado Classic 2007 Owner's Manual page 476

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:
{
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.
If your vehicle is a dual fuel tank chassis cab
model, and it runs out of fuel, refuel the front fuel
tank first to ensure a quick restart.
476
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 595.
When replacing the fuel cap, turn it 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 258.
The TIGHTEN FUEL 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 269 for more
information.