Sparing the details of why, I replaced the fuel tank in my 2000 TJ with a used model. Once done, I found that I could'nt fill it past the 1/2-full mark. The pump simply won't allow it, and the fuel will run out the filler-neck if I try to fill the 2nd-half of the tank using a 5-gallon tank typically used to fill a lawnmower. My old tank, original vent lines, original pump, etc. never had a problem allowing me to fill it to 100% full (at least according to the fuel gauge on the dash) and none of those parts were replaced during the swap.
Tonight, I dropped the replacement tank to take a look around again. Nothing is obvious yet, however I made a few comparisons to my original tank that I still have. Can anyone tell me the purpose of the "ping pong ball" that sits inside the tank's nipple on the filler port? I noticed that my original tank has one that's spring-loaded, however it appears that the replacement tank's "ping pong ball" is not spring-loaded (or is broken). Curious to know if this might be my problem.
Well the tank is probably full if even the 5-gallon tank cant fill it anymore. This means either the sending unit was damaged when you switched the tanks or it simply isn't compatible with the new (used) tank.
Here's a way to test it. Either drop the tank to get to the sending unit or take out the gas gauge and get to the wires that come from the sending unit. One will be negative or ground and one will be positive and will go right into the sending unit.
Use a voltmeter to measure the Ohms of resistance between the wires that run to the sending unit and ground. With float in up position, resistance should be 20 ohms. With float in down position, resistance should be 220 ohm. If it reads close to 220 ohms but the gauge only reads half, you probably have a problem with the gauge. If it reads something like 100ohms, then the gauge is probable fine and its probably something wrong with the sending unit or the tank itself.
As far as the ping pong ball, I can only speak for chevy sending units, but I assume they are all similar. In my old chevy truck, their are 2 lines on the top of the sending unit. One that feeds the engine, and one thats a vent. The little pingpong ball is on the line for the vent so if gas splashes up (like of you are going up a steep hill), the little ball will plug the line and stop gas from going out the vent. When you are level again, the ball should drop and allow gasses to enter or escape. It is possible you are having "Vapor Lock" in the tank because the vent could be plugged. Again, I havent seen a TJ fuel sending unit before but it should be a similar idea.
You said it was used; are you sure you didn't get a 14 gallon tank from someone? They look identical from the outside, but the 14gal one has an extended internal fill tube that restricts the fill level. The good news is that you can turn it into a 19 gallon tank by cutting the stupid inner tube shorter.
Anyone have experience with replacing only the sending unit? My pump seems to work fine and this would be a lot cheaper. The discount auto stores appear to only offer the pump, float, sending unit, etc. as a complete assembly.