I need to know how to replace my fuel sending unit on my 98 Volvo s90

These Isuzu-built vehicles had many problems with fuel system components. Particularly the sending unit in the tank. Note: If the unit is replaced, the PCM must be updated to accept the new pump calibration before it will work.

What You'll Need:

• flat head screwdriver
• Phillips head screwdriver
• hammer
• open end wrenches
• replacement fuel tank sending unit
  Before you begin, disconnect the negative battery cable to be sure no electrical sparks are possible. You're dealing with gas which is highly flammable!Your fuel tank sending unit is located in the top of the fuel tank, but is accessible under your back seat (or under the carpet in your trunk) on some models. The sending unit will be protected by an access cover, usually held on with a couple of screws. Lift your back seat or trunk carpet and locate the access cover for your fuel tank sending unit. Remove the screws holding the cover in place and remove the access cover to reveal the fuel tank sending unit.
  Safety Note: Gasoline is highly flammable. By removing the fuel tank sending unit, you are opening the gas tank. Be sure there are no sources of spark or flame nearby.
  With the access cover removed, you'll see the fuel tank sending unit right there on top. It will have a wiring harness plugged into the top (this tells your gas gauge how much fuel is in the tank). Unplug the wiring harness and move it safely to the side. If your fuel tank sending unit is screwed or bolted in place, remove the screws or bolts. Some sending units are a "twist-lock" type. They work like the old twist-lock gas caps. You'll see a few notches along the outer ring of the sending unit. Place the tip of a sturdy flat head screwdriver in the notch and gently tap it counter-clockwise. The sending unit will rotate until it's loose. (the picture above shows the fuel tank out of the car to illustrate the fuel tank sending unit's location). Now you can remove the fuel tank sending unit in one piece. Attached to it is a long rod with a float at the end, so you might have to try a couple of different angles to get it out. As usual, installation is the reverse of removal. Don't forget to plug the new sender in or it won't send you anything!
  
  

It is located in the gas tank as a part of the fuel pump assembly and im not sure about the access but if you cant see a removable plate in the trunk then the tank will have to be droped.

it is inside the gas tank, part of the sender unit. you have to take the tank out of the car in order to replace it. while it is out of the car do yourself a favor and replace the fuel pump too so you do not have duplicate labor when the pump goes out next year. before you go nuts, pull the liner or carpeting out of the trunk and make sure your car's trunk does not have a service access to the gas tank.
generally gas tanks are mounted under the trunk.

No I dont think you can as I cant on my 98 oldsmobile achieva and they are basically the same car. Almost all parts are interchangeable between the two.

91, 92 and 93 Honda accord sedans have a tank access hatch which allows you to remove the sending unit with out dropping the tank. It is located under the trunk carpet behind the drop foward rear seat back. on the passanger side.
I suggest the battery be disconnected before you attempt this. If you have a factory equipt radio, you will need your code to reset it after reconnecting the battery.
Tools needed brass punch (to prevent sparks and explosions from the striling hammer), hammer, phillips screw driver toothe brush or equivilent cleaning brush..
I opened all doors and trunk for ventilation. Put the rear seat back down. The trunck carpet folds foward to the front of the car. You should be able see the access hatch with three phillips head screws. Remove the screws and lift te hatch carefully not to dammage the wire harness. Remove the wire harness fron the sending unit. Pull rubber protector back to expose the harness connecter. Clean this area good with the brush. Dirt in the tank will cause other preformance issues with your vehicle. With the brass punch rotate the locking ring 1/3 of a turn to allow it to dissengage it. lift the locking ring out and place aside. Lift the sending unit while twisting it getly counter closkwise 1/4 of a turn. gentlh minipulate the sending unit out of the tank by assisting the float thru the opening. Replace in reverse order.

Inoperative purge valve in EVAP system

Disconnect battery ground strap before beginning to work on the fuel/electrical system. Do not smoke or have anything in the area which can ignite or cause sparks

1. Drain enough fuel from tank to prevent spilling, no more than 4 gallons (16 liters) of fuel should remain. Use suitable pumping equipment.
2. Remove floor mat and panel from trunk.

1. Disconnect wiring harness connector to sending unit. Remove wires from connector.
2. Disconnect ground wire to sender. Push wires through grommet.
3. Remove cover for access to tank sending unit.

1. All models: Disconnect from the fuel tank sending unit:

1. Breather hose C
2. Return hose A
3. Supply hose B
4. Filler hose D
5. Wiring harness from clip E

1. Clean area and unscrew fuel tank sender/pump cap or equivalent. Remove tank sender/pump and seal.
2. Remove pump from sender/pump assembly.

INSTALLATION

1. Inspect the pump for damage, feed pump hose and wiring for wear. Replace as needed.
2. Install new pump to sender/pump assembly.
3. Install rubber seal in neck of tank. Lubricate inside of seal.

1. Install fuel tank sender/pump unit. Align it with the moulded joint (arrows) on tank.
2. Apply rustproofing compound to exposed metal portions of tank sender.

CAUTION : With tape or rubber caps, protect fuel sender openings from overspray.

1. Torque fuel tank sender/pump lock ring to: 22 ft-lb (30 Nm)
2. Reassemble components in reverse order. Use tie straps to secure all hoses and wiring.

You do need to replace the guage sending unit in the gas tank, u must remove the gas tank, this is not really a job for a DIYer.

you will need new sending unit you have 2 left and right you will need a scan tool and one is not working right most the time is left side sending unit bad not working it can be replaced from inside