Ford Ranger Owner's Manual page 267

If you lose the fuel cap, replace it with an authorized Motorcraft
or equivalent part.
If you do not use the proper fuel cap, the pressure in the
fuel tank can damage the fuel system or cause it to work
improperly in a collision.
NOTE: If you replace your fuel cap with an aftermarket fuel
filler cap, the customer warranty may be void for any
damage to the fuel tank and/or fuel system.
If the fuel cap is venting vapor or if you hear a hissing
sound, wait until it stops before completely removing the
cap.
You may occasionally hear and/or feel a slight thump occurring
one or two seconds after a gentle stop. This is normal and
generally occurs with the fuel tank 3/4 full. It is caused by the
wave action of the fuel within the tank.
A venting fuel tank is not an abnormal condition. It may be
caused by:
Too volatile a fuel for the weather conditions. The service
stations sometimes sell winter grade fuel in the spring.
Pulling a heavy load on hot days, or at high altitudes.
Extended periods of idling with the engine RPM increased
above the normal idle range.
Parking vehicle in full sunlight for extended periods on
extremely hot days.
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