Going to reach out to @heymccall as I recall he knows these old late 90s NPRs and probably has this figured. The fuel gauge be whacked out. Fill it all the way up, I think it likes 1/2 tank. Some days you fire it, says super low fuel, then starts to dream about more fuel as you drive a bit. I'm just curious the best place to start? Is this maybe a sender wire contact at the tank? Hoping not the actual sender in the tank. I don't have specs for that to measure it. Could fill and test, but wasn't sure if there is a likely culprit before I dig in?
Odds are it's the tank sender. What tank is on yours, side mount or between the rails? I'm thinking the sender is a 240ohm empty, 33ohm full, unit regardless of which style tank. It's been a while, but, I think there was a chassis plug, forward of the tank, where the sensor harness could be unplugged.
It's a between rail tank. If memory serves, just rear of the rear axle. Square looking tank. I can poke around but figured I should first check in. Not yet sure what is going on as it seems to change it's mind while driving. I will poke.
Well..... I did what I could for now. Truck is actually a 2003 diesel with 3 wire sender. Unplugged sender from harness and grounds all seem to check out. Sender ground wire to tank, tank to chassis, harness ground to tank, etc. Power to sender shows 11V. Shorted sense wire to power in harness for test and it pulled down to 9V at 35mA, pretty much looking like current limiting, but that gauge wanted to live at E regardless. Open closed, even ran a potentiometer between then at 20-1.5K, nothing got the needle to move on the gauge. I plug the sender back in, poof, moves right up to 1/4tank, which is probably close. I'm sure something is off with the sender, but I just like to confirm things. I wanted to get that gauge to show full. Guess I am missing some circuit information as I figured that was a simple voltage divider. Surely it can't be PWM....?
Isuzu will give you access to their service portal for $75 for thirty days. Their site is easy to navigate tofind the needed info, I'm impressed. You can't save the manuals but you can save screen shots of the pages you need. https://www.isuzutruckservice.com/login.php?type=Aftermarket
Thank you for that information!!!