Decided to come up with a way to test the gas gauge, beyond the shop manuals simple but flawed way.
If your gas gauge doesn't work at all or reads low, the shop manual instructs to disconnect the feed wire at the back of the trunk wall, behind the filler door. This will cause the gauge to spike to past full. Then ground the connector, and the gauge will spike to empty.
On my own car, this proved to be a false indicator of a good gauge, My gauge read about 1/4 tank low, at all times. Swapping the sending unit didn't work. I swapped a gauge, and it worked, and has worked for a while now.
This is how I devise a test for the gauge.
The sending unit rheostat reads at 1.5 to 30 ohms. I picked up several different types of resistors from radio Shack for some testing. Here is what I found;
30 ohms, resistor reads full on a gauge.
15 ohms reads 1/2
10 ohms read 1/3 tank.
I have several gauges, so I have backed up the readings.
I then tested the known bad gauge. (I dont throw away anything) The gauge read low, like it had in the car. remember that it passed the shop manual tests.
The packs that I picked up were 10 ohm and 15 ohm 1/2 watt. I soldered the 2 15 ohm resistors together to create the 30 ohm.
So, with that said, get your self a 5 pack 15 ohm 1/2 watt resistors from Radio Shack. Since your gas tank is already out, put the resistors in line at the feed line, where it hooks up to the gas tank sending unit. Move your way up, until you have found the problem. I believe I twill be a bad wire, or a bad gauge. If the gauge even reads 1/8 low, with the resistors, the gauge is bad, or a bad wire.