The fuel pump on your Subaru Impreza needs little or no service at all. However, years of service wear out internal components, which affect fuel delivery pressure and engine performance. Fortunately, changing your Impreza's fuel pump is a very simple task following these steps. The procedure applies to the 2004 model, which is very similar to other years.
Park the vehicle in a safe and level place.
Open the bonnet and start the engine.
Disconnect the fuel pump relay located in the power distribution box or fuse box at the front and to the left (driver's side) of the engine compartment. Markings on the relay, the inside or top of the distribution box lid should identify the relay.
Wait until the engine stalls and crank it for about 3 seconds to relieve the remaining pressure in the system and turn the ignition off.
Loosen the fuel filler cap, raise the rear seat and move the floor mat to the side.
Remove the access lid on the floor to expose the fuel pump assembly on the fuel tank and disconnect the fuel pump electrical connector.
Clean the fuel lines connectors and around the fuel pump assembly lid to avoid fuel system contamination.
Remove the jet pump hose and disconnect the two quick connectors from the fuel supply and return lines. Hold the quick connector and press down on the locking tabs at the end of the connector; then pull the line to disengage the connector. Place a shop rag underneath the fuel lines to absorb any fuel remaining in the line.
Remove the nuts holding the fuel pump assembly to the fuel tank using a wrench or ratchet and socket.
Lift the fuel pump assembly off the tank, discard the O-ring seal located underneath the assembly lid and install the new pump on the assembly.
Install a new O-ring seal, set the fuel pump assembly in the tank and tighten the nuts in a crisscross pattern. Torque nuts to 3.3-foot lbs. (4.4 Nm).
Install the lid fuel hoses, plug the pump electrical connector and fuel pump relay. Start the engine and check for leaks. Then install the access lid, floor mat, rear seat and tighten the fuel filler cap.
Fuel is extremely flammable. Do not allow smoking or park your car near appliances with open flames, like water heaters or dryers, whenever you have to work on the fuel system of your vehicle to avoid a possible accident.