1996 Ford Explorer Owner's Manual - Page 341

Servicing Your Explorer 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. 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: s Too volatile a fuel for the weather conditions. The service stations sometimes sell winter grade fuel in the spring. s Pulling a heavy load on hot days, or at high altitudes. s Extended periods of idling with the engine RPM increased above the normal idle range. s Parking vehicle in full sunlight for extended periods on extremely hot days. 341