GMC Hummer H3 2008 Owner's Handbook Manual page 249

Service and Maintenance
After unlocking the fuel cap,
remove the fuel filler cap by push-
ing the fuel cap down and turning
it slowly anti-clockwise. Replace it
by turning it clockwise. When you
put the cap back on, continue to
turn it clockwise until you hear a
clicking noise.
If you drive your vehicle with a
loose fuel cap, fuel may evaporate
into the atmosphere. If this
happens the malfunction indicator
light may come on.
Check the fuel cap to be sure that
it is properly installed.
. . . 248
CAUTION
Fuel vapour is highly flamma-
ble. Keep sparks, flames and
smoking materials away from
fuel.
If you get fuel on you and then
something ignites it, you could
be badly burnt.
Turn off your engine before
refuelling.
Always switch off the engine of
the vehicle and any mobile
phones when refuelling.
To avoid injuries to you and
others, read and follow all
instructions on the petrol
station pump.
CAUTION
If you open the cap rapidly
while pressure inside the tank is
high, fuel may spray out from
the tank. Before you fully open
the cap, turn it slightly to
release the pressure from inside
the tank.
If you hear a "hiss", wait for that
to stop. The "hiss" means there
is still some pressure left. Then,
remove the cap.