How is your engine going to run if the gas can't get to it? That's where the fuel sending unit comes in. In addition to moving the gas from the tank to the engine, the sending unit also measures the amount of fuel in the tank and supplies that information to the driver via a gas gauge. Few things are more frustrating to a driver than not knowing how much fuel is in the tank. If the gauge is always reading the same then it could be time for a new fuel sending unit. The fuel sender is located in the tank and can often rust or break given time. The locking ring and gasket can that hold the part in can also break. The sock that filters the fuel going into the unit can become clogged or tear allowing unfiltered fuel into the line. The fuel sending unit float is the part that moves up and down as the tank fills or empties to measure the amount of fuel.
The fuel sending unit does what you would expects, sends fuel from the fuel tank to the engine in conjunction with the fuel pump. Just as importantly it sends information to the driver about the amount of fuel in the tank. These units are irregularly shaped and designed to fit into the specific tank in your car. The sender is usually made out of metal and includes a mesh sock for a filter and a float to measure the fuel. The float is foam or sometimes brass. The float drops gradually as fuel leaves the tank. This set up can lead to inaccurate gauging of the fuel amount but still gives the driver a good idea of how much fuel is left. Before the fuel sending unit and gas gauge, drivers would have to guess how much fuel was left or use a dipstick. Some early tanks had a glass covered clear opening that allowed the driver to actually see into the tank. Thankfully, we now have the fuel sending unit and gauge to warn us when the tank is getting low.
As with anything dealing with gasoline, changing a fuel sending unit can be dangerous. It takes several steps to replace one. If one is uneasy with the task, it may be best to seek a mechanic.