I have a 1967 Impala SS about two years ago it started. My fuel gauge constantly reads full tank, thanks to this problem I've run out of gas twice. I started testing at the power or positive line coming from my wiring harness thru the trunk on my car, it's reading 9.91 volts and I also checked the ground wire under the car to make sure it was well grounded. I noticed that when I unplug the positive line my fuel gauge goes well beyond the full mark (it's pointing completely to the right) and when I plug it back in it returns back to the full mark. Is this a problem with my gas gauge or my sending unit?