How do you replace the fuel filler neck for a 2004 lesabre?

Jack up the rear of the car. For safety's sake put jack stands supporting rear of car. Remove driver's side rear wheel. Pop of small plastic door around filler tube from inner fender( it just snaps back in place). Loosen hose clamp that secures the filler tube to the rubber boot inlet to fuel tank.

The top of the old filler neck is supported by a large round rubber grommet.where it sticks through the round hole into the filler area behind the filler door. This grommet is rubber, but has a steel band inside inside of it. This grommet just slides over the filler neck. Unplug the wire from the metal end of the tube that is plugged into the metal rim ( this is a ground for static electricity when you are fueling the car. Don't leave this out when re-assembling )

See if you can now push the old tube out( the grommet may push out staying on the tube ). If it will not come out, you may have to loosen the grommet by working on it up top to loosen it, and then pull the grommet off from the end of tube. Don't destroy it. It will need to be re-used.

Take the grommet, and push it onto the top of the new filler tube until it will go no further. Take some oil or transmission fluid, and coat the end of the filler tube where it will go into the lower rubber boot where the bottom of the filler tube goes into the gas tank. Also coat the inside of that rubber boot. Feed the new filler tube in from the top, twisting it some back and forth as you push down on it forcing into that lower boot, until that grommet fits snugly back into the hole supporting the top of the tube. Re-attach the static ground wire to the inside of the fender. Once the tube is fully seated in the top grommet, and the bottom of the filler is down into the rubber boot, tighten the hose clamp to secure the tube to the boot. Before you tighten that clamp, make sure the hole where the filling station nozzle goes into the tube, is on the bottom 6 o'clock position. Snap the little plastic door back on where the filler tube goes through the inner fender.

Do not force the tube when working with it, It is made of some sort of polyethylene, which is famous for nothing sticking to it very well. The top of the tube is a separate piece that is used on all kinds of other cars, so it is plastic welded, or glued to the tubes for specific applications. Any kind of glue or plastic welding on this material will not be a very strong bond, so treat it like it is a little fragile, because it is.. Odds are pretty good that many people were told that their initial problem was the gas cap, so they bought an aftermarket gas cap. I HIGHLY recommend either using the original GM cap if it still seals well, or buying a new one off of our favorite internet auction site. Again, this is because of the weak point where the top of the tube is attached to the rest of the tube body. The ratcheting action of the oem cap is much lighter than most of the aftermarket caps, and slips on and turns easier. I found that to be true, and that is why I was privileged to do this operation twice.