The fuel tanks are located at the rear of the vehicle. The tank itself also contains a fuel gauge sending unit, and a filler tube. In most tanks, there is also a screen of some sort in the bottom of the tank near the pickup to filer out impurities. Since the advent of emission controls, tanks are equipped with a control system to prevent fuel vapor from being discharged into the atmosphere. A vent line in the tank is connected to a filter in the engine compartment. Vapors from the tank are trapped in the filter canister, where they are routed back to the fuel tank, making the system a closed loop. All the fumes are prevented from escaping to the atmosphere. These systems also require the use of a special gas cap which makes an airtight seal.