The fuel sending unit is responsible for sending fuel-level information to the power train control module and your fuel gauge. Excessive carbon in gasoline can build up on the fuel sending unit, causing it to malfunction. Ford recommends adding a bottle of Techron fuel system cleaner to a full tank of fuel to resolve the issue. If you have done this twice and still have an issue with the sending unit, replacement will be your only choice.
Disconnect the negative battery cable.
Determine which style of gas tank your Mustang has by researching it in the shop manual so that you know whether to access it from underneath or by removing the back seat bottom. You can pick up a Haynes manual specific to your Mustang at most auto parts stores or head down to the public library and thumb through the full size Chilton manuals for specifics.
If you have a newer model, you will need to access the top of the fuel tank by removing the back seat bottom cushion. If this pertains to you, go to step three.
If it is an older Mustang, you will need to drop the tank down to the ground to access the pump. If this describes your Mustang, go to step four.
Put on your safety glasses and reach into the back seat. Slide your fingers carefully under the lip of the seat bottom and firmly pull upward. The seat bottom should pull free with some considerable force. Once you have the seat bottom free, remove it from the car and place it someplace safe. Now, go to step nine.
Put on your safety glasses and climb under the rear of the Mustang. The fuel tank is held in place by two long support straps.
Raise the jack to the center of the fuel tank to support it. Then, remove the bolts holding the support straps in place.
Use the socket set to remove the bolts holding the straps in place. There should only be one bolt on each strap, located at one end. The other end of the strap slides into a mount that you can remove it from, if desired, after the tank is down.
Slowly lower the tank with the jack, until you can reach up and disconnect the fuel lines from the pump on top of the tank with the fuel line disconnect tool. The filler neck will only allow you to drop the tank a few inches, so jockey the tank somewhat to give you access from the side opposite the filler neck. At this stage, you will also want to disconnect the wiring harness for the pump and sender by unclipping them and moving the harness carefully off to the side.
Unbolt the filler neck retainer and carefully slide the filler neck out of the tank. It might be best to have a helper for this. Once the filler neck is out, pull the tank back and away from the vehicle.
Wrap a rag around the tip of the screwdriver and place the screwdriver against the lock ring on the top of the tank where the fuel pump enters. Carefully tap the screwdriver with the mallet to pop the lock ring free.
Remove the fuel pump by carefully lifting it up and out of the tank. The sending unit is part of the pump assembly and is not serviced separately, so you will need to swap out your existing pump assembly with the new one. When you install the new pump, be careful not to pinch the new gasket, as this can cause a fuel leak.
Reverse the steps to reinstall pump, tank, fuel filler neck, wiring harness, tank, etc. When done, reconnect the negative battery cable and start the Mustang, then carefully inspect for any leaks around the tank and filler neck. If you see any, turn off the car immediately and fix the leaks.
