Ford Mercury Villager 1997 Owner's Manual page 311

If you lose the fuel filler 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.
The fuel system may be under pressure. If the fuel cap is
venting vapor or if you hear a hissing sound, wait
until it stops before completely removing the cap.
NOTE: If you replace your fuel filler cap with an aftermarket
fuel filler cap, the customer warranty may be void for
any damage to the fuel tank and/or fuel system.
A venting fuel tank is not an abnormal condition. It may be
caused by:
too volatile a fuel for the weather conditions. 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 the vehicle in full sunlight for extended periods on
extremely hot days.
Do not overfill the fuel tank. The pressure in an
overfilled tank may cause leakage and lead to fuel spray
and fire.
Maintenance and Care
WARNING
WARNING
WARNING
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