Re: WTB: fuel tank sender unit - '91 CV 5.8L
You're probably facing the same problem I faced a little over a year ago - the item is no longer produced by Ford. I even called around the country (Canada for me) and came up with nothing.
In my particular case, I had purchased an aftermarket fuel sending unit (new) from NAPA. However, it is of extremely poor quality, and there were no other manufacturers to choose from. The first sending unit I purchased leaked from the plastic electrical plug. I was able to return that, and tried another new sending unit - only this one spiked the fuel guage all the way past "Full" and it stayed there.
What's the saying - necessity is the mother of invention? Anyway, I decided to diagnose my old sending unit (there were times when it'd read empty when the tank wasn't, and for a while it'd spring to life - and cleaning all electrical connections did nothing to resolve the issue). Again, in my particular case, I discovered the original sending unit that was put on the car when it rolled off the assembly line had a bad float. It's made of tin - very thin tin. It had a small crack in it, and I only noticed it because a day later there was wet gasoline on my bench. Naturally, depending on temperature, this crack would expand and let in fuel, and the float would sink.
In my case, I kept the aftermarket sending unit, and robbed it of it's float. However, that was both a lazy & expensive venture. You CAN repair your old float any number of ways. However, the best way would be to solder the leak.
I can't say for sure that you have the same problem, but you'll have one of two conditions with the original sending unit - it'll either have a bad float, but more common, you probably have bad ground (due to corrosion at the sending unit). Clean all of the electrical connections, including the ground connections (this includes the locking ring for the sending unit). If the unit is already out of the tank, inspect the float carefully.
As an aside, the quality of the aftermarket sending unit compared to the factory original is like night & day. The original Ford sending unit will last a very very long time. It's a very simple component, and they aren't difficult to repair. I'm probably good for another 20 years.