1996 Ford F250 Owner's Manual - Page 311

If you lose the fuel cap, replace it with an authorized Motorcraft or equivalent part. RWARNING 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. RWARNING 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: u Too volatile a fuel for the weather conditions. The service stations sometimes sell winter grade fuel in the summer. altitudes. u Pulling a heavy load on hot days, or at high u Extended periods of idling with the engine periods on extremely hot days. RPM increased above the normal idle range. u Parking vehicle in full sunlight for extended 312