Ford Vehicle Owner's Manual page 311

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.
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 summer.
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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