Fuel lines are usually made of metal tubing or synthetic materials.
A fuel supply line carries fuel from the tank to the engine. A return line may also be provided to allow excess fuel to return to the tank. This helps prevent the formation of vapor that can occur in the fuel supply during hot conditions.
The fuel filler is where fuel enters the system. The filler neck is a pipe that extends above the fuel tank.
On unleaded gasoline vehicles with catalytic converters, the filler neck is designed to prevent leaded fuel being added. Its diameter is smaller than those on leaded vehicles, and a trapdoor inside the filler can only be opened by the nozzle of the unleaded gasoline spout.
The filler cap seals the filler neck to keep out water and foreign objects. Water can corrode internal passages of fuel pumps and carburetors.
The cap on the gasoline tank also stops gasoline vapor escaping and polluting the atmosphere. This is important for gasoline which is very volatile and vaporizes easily, especially in warmer climates.
Some gasoline caps have a low pressure valve built-in. It keeps a balance between the pressure in the tank and the outside atmospheric pressure.
As gasoline in the tank is used, the air space above the fuel increases. This causes a fall in pressure, compared to outside atmospheric pressure. The valve then opens and lets more air into the tank.
A fall in temperature can cause a fall in the pressure in the tank. The valve opens to admit more air till inside and outside pressures equalize again.
The fuel gauge shows how much fuel is still in the tank. It has a gauge unit on the dash, and a sender unit in the tank. This unit indicates the level of fuel in the tank, and transmits this information to the gauge in the dash-panel.