Fuel systems start with putting gas in the tank and there are a lot of fumes associated with all of that gas.
Because of that, newer vehicles have systems that are designed to keep the gas fumes from escaping out of the tank into the atmosphere where they would cause pollution. In order for that system to work, everything needs to be sealed up tight and monitored after you put the gas in. That’s why you might get a check engine light on if you leave your gas cap loose after you fill up.
Gas in the tank is pushed towards the engine through an electric fuel pump. These pumps produce a precise amount of pressure and even a minor drop in that pressure can cause major problems such as a rough running engine, stalling and failing to start. Many times a fuel level sending unit is built into the fuel pump assembly as well and it sends a signal to the fuel gauge on the dash showing how much fuel is in the tank.
Fuel filters are an important part of the fuel system because they trap tiny contaminants in the gas that might plug things up like the fuel injectors. To avoid problems, have these replaced as recommended by the manufacturer.
Fuel lines run under the vehicle and carry the fuel up to the engine. Since the fuel is under pressure as it goes through these lines, any sort of problem with a damaged line or a clamp can cause a leak to appear. Have your mechanic take a look at your fuel lines for signs of severe rust or damage from running over something in the road.
Once the fuel has traveled through the lines, it heads up towards the engine and into a fuel rail. The fuel rail carries the gas to the individual fuel injectors on the engine. Fuel injectors are the end of the line for the fuel and they are located at each cylinder on the engine. They spray a fine mist of fuel that mixes with air and is ignited by the spark plugs. The openings where the fuel comes out of the injectors are very small, so make sure that you maintain your fuel system to keep everything clean.
There also are electronic sensors related to the fuel system and if these sensors start to act up, they will send the wrong information to the computer. When that happens, the computer might command the fuel injectors to spray the wrong amount of fuel into the engine, resulting in bad gas mileage.
If you have a problem with any of the parts in your fuel system, your gas mileage might be a fraction of what it used to be. Get the problem fixed and you should have less of a “jump” when you go to the pump.