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Most of the time it's the float level sending unit inside the tank. The contacts wear out. It's not too hard to change. A new sending unit is like $50, but you'll have to transplant your fuel pump onto the new sending unit.
Or you can get a new fuel pump with the sending unit together for like $80 or $90
Going through the back seat to trunk access, peel back the carpet at the back of the trunk and you'll find a access panel to the fuel pump. The access panel is held down by several 10mm bolts ( if I remember correctly ) and the panel itself is slightly glued with adhesive/sealant to the trunk floor, so you'll have to pry it up a bit with something like a screwdriver.
Once the panel is off, you'll see the top of the fuel pump. Ahh hell.... just youtube " Buick fuel pump replacement" or "3800 fuel pump replacement".
and a few other cars are the same procedure.
Welcome to the forum !
I had a 98 Regal and I still miss that car ! I did mod it a bit. Enough to get down to 14.3 @ 97 at the 1/4 mile. Not supercharged or anything... All motor and bunch of stuff from zzp and my own tune with HP-tuners.
Now I drive a 2018' Regal with the 2.0 L 4-banger turbo that and ran it down the 1/4 mile in 14.3 seconds @ 96 mph 100% the way it came from the factory !
A relatively cheap alternative which may or may not work is a bottle of Techtron Plus, black bottle. There appears to be a common problem with modern cars and the sulfur content of gasoline. The contact in the sender unit has a silver coating and the sulfur bonds to the silver causing erratic readings, full, empty, bouncing etc. You will find many people have resolved the issue by adding the product to a near empty tank and then filling up.