I have a 98 gmc and the gas guage does not work properly. It stays on half tank and when gas gets real low, it begins bouncing back and forth erractially.

 

i would check the wiring to the tank first, could be a loose ground.

 

lotsa possibilities,but probably the sender in the tank

 

I have an 01 that does the same thing. Except it really freaks out when I fill up. It only reads at the correct level 1/4 of the time. I have not gotten under to check the wiring yet .

 

could be the sending unit. It wont be easy changing it if it is.

 

Why not? Dropping gas tanks is a breeze with 2 people and a big shop jack. For God's sake don't take it to the dealership! I went to the Chevy dealer the other day and they said it would run my $800 to change the fuel pump in my truck if that ends up being what's wrong with it! Me and my dad changed the one in his 2000 Sierra ourselves and the pump only costed a little over $250 with a small discount at O'reilly's.

 

Try checking the fuel pump, that did the trick on my uncle's truck!

 

My 1999 had the same issue, changed fuel pump/sending unit module and it solved the problem.
I pulled the box off my truck, pretty simple, except for the damned lock ring!
Bought my unit from Hamilton Chevrolet in Detroit for $175, they are a seller on ebay.
Chevron Techron fuel additive is supposed to prevent the proble, dont know if it will fix it though.
Sulfur in the fuel attacks the sending unit contacts and creates the problem.
Additive is available at NAPA, $10/pint