my 1991 nissan sentra twin cam 1.6 gas gauge goes up and down when tank is filled up can anyone tell what's causing this problem and how to fix it.

What do you mean by going up and down?

when i fill up the tank all the way the little gas gauge is all the way up and then when i start driven all of the sudden it starts going down really fast till half tank and then goes back up till full and keeps doing the same thing.

sounds like the sending unit is bad or you have a wiring problem; perhaps a loose ground or open circuit.