When i'm in park/neutral my gas gauge goes back and fourth to full and empty.. But when i put it in drive and drive down the road for a few miles it starts to tell the truth.. What is that? A bad sending unit?

 

sounds like the sending unit. mine did this right before the fuel pump died on me. best of luck to you. might wait to see what some others around here say who are more knowlegable than me.

 

I agree, fuel level sender is probably failing. It can be serviced separately from the pump, but as mentioned, the pump is likely to fail shortly also.

 

The fuel sending unit isnt the only thing that controls the fuel gauge. The signal goes through the fuel sender buffer and the vehicle control module. My guage likes to go low as soon as i get down below 3/4 of a tank. Stick a meter on the sending unit and the resistance reads just like it should. There is a whole bunch of voodoo that goes on in the vehicle contorl module. Is uses 5vdc reference voltages all over the place. I suspect that my reference voltage isnt constant, I currently suspecting the little voltage regulator that creates this 5 volt reference.

Replacing a fuel sender isnt fun, easy, or cheap. check it before you replace it.