Fuel injected vehicles are designed to supply more than an engine needs with the excess being returned to the fuel tank. Your GM W-Body vehicle has both fuel feed and fuel return pipes to carry fuel from the fuel sensor assembly to the fuel injection system and back to the fuel sender assembly.
Many vehicles now use nylon fuel pipes, designed to perform the same job as steel or rubber fuel lines used in older vehicles. Care is required when working around the fuel feed and return pipes. Nylon pipes are constructed to withstand the maximum fuel system pressure, exposure to fuel additives and changes in temperature. Two sizes are used: 5 / 16 in. for the fuel return and 3 / 8 in. for the fuel feed.
The fuel feed and fuel return pipes are assembled as a harness. Retaining clips hold the fuel pipes together and provide a means for attaching the fuel pipes to the vehicle. Sections of the fuel pipes that are exposed to chafing, high temperature or vibration are protected with heat resistant rubber hose and/or corrugated plastic conduit. The fuel feed and fuel return pipe connections are sealed with replaceable O-ring seals. These O-ring seals are made of special material and should only be serviced with the correct service part.
Nylon fuel pipes are somewhat flexible and can be formed around gradual turns. However, if forced into sharp bends, nylon pipes will kink and restrict fuel flow. In addition, once exposed to fuel, nylon pipes may become stiffer and are more likely to kink if bent too far. Special care should be taken when working on a vehicle with nylon pipes.