Fuel gauge problem
 

67. GTO, just put a new gas tank in, put in a new fuel sending unit, Key On, Tan wire not connected at tank, fuel gauge reads full, Connect tan wire at tank sending unit, gage goes to empty. Put 5 gallons of gas in it gauge still reads empty. This is the second fuel unit I put in. I ran a bit check on the one I just put in and the ohms goes up if you left the float, what the heck can be the problem and yes, it’s fully grounded.
Thanks for your help,
LET

always......always test the fuel gauge out BEFORE you put fuel in it.
you may need to temporarily extend the ground wire but connect
wiring and manually move the float arm to test
If you have an ohm meter it should read 0-90 ohms going in a full sweep.

Unlikely, but worth checking, remove the sending unit from the tank and look inside to see if anything like a baffle or something might be stopping the float from traveling. For that matter, run jumper wires for ground and tan wires to allow you to submerge the sending unit it in a container and add fuel to it to make sure the float actually floats. Remember, you're making it work outside the car by moving it with your hands.

One more thought. I too have a 67. I switched mine over to fuel injection and replaced my sending unit with an in tank high pressure fuel pump. I believe I was able to siphon all the gas out of the tank before I installed the new unit, and the old sending unit and the gauge were spot on. When I poured 5 gallons in, the gauge said I had between a quarter and a half a tank. So either the tank wasn't completely empty, or the rod on the float needs to be adjusted slightly by bending it to get it to read more accurately.
My point being, if 5 gallons doesn't move or submerge your sending units float, its the wrong unit, or it needs to be adjusted. The instructions may mention this, or check with the company that supplied the sending unit.
Good luck and keep us posted

If you still have the old sending unit test it before pulling the tank again. I wouldn't think it would hurt to submerge it in a bucket of water to see if the float moves. You could also use extension wires to ground it and run a positive feed to the gauge.

I agree, that's why I said to submerge it in a container and add fuel to it. Never put it in water.