Pretty sure my FSU is malfunctioning. When filled above a quarter tank my gauge becomes erratic, jumps to three quarters, drops near E, etc, and eventually finds its way back to the proper reading.
My understanding is the most common cause is sulfur deposits having accumulated on the potentiometer (aka variable resistor). My plan is to attempt cleaning the device rather than immediately replace because of the crazy cost of such a simple device. Ford OEM is ~$370, and aftermarket (Delphi) looks to be ~$260. If cleaning fails, or works then fails shortly afterward, then I suppose I'll have to replace. In that case, I would probably just go aftermarket since they both have the same design prone to failure anyway (potentiometer spends its life stupidly soaking in diesel).
So then, how do we clean it? I did a small amount of research to find which easily available
chemical solvents work best on sulfur. Sulfur in case you aren't aware isn't prone to dissolve in much of anything. First thought was vinegar (which works miraculously on most things) aka dilute acetic acid, but reportedly has very minor effect on sulfur. Other options are brake cleaner which is methyl acetate ("nail polish remover") w/ some hydrocarbons mixed-in, MAF
sensor cleaner (n-Hexane), and acetone. Apparently n-Hexane and acetone can both dissolve sulfur. Solubility of sulfur increases w/ temperature. My idea is to soak the potentiometer in a warmed bath of acetone. Not sure about fumes, etc, but I will keep this far away from people while it soaks.
As for design improvement of the FSU, the potentiometer itself really should be isolated from the diesel fuel in some kind of sealed sleeve. The "wiper" design doesn't make that possible far as I can see. I really wish there were versions of the Metrum Rod FSU (sorry, my acct isn't old enough to post links yet) invented by "Leroy Diesel" designed for the various Ford Super Duty tanks... so far his only model is designed to be plug-and-play for some Chevy vehicles.