1987 5.0 Firebird that I am about to light on fire. Recently I had a fuel leak coming from where the tank and nozzle meet. Had to drop the tank a little to fix it. Once it was back up and running, I noticed my gas gauge was past full. After troubleshooting (sending unit reading 140 ohms), confirming the gauge worked, I dropped the tank and replaced the sending unit. I think I messed something up when i syphoned the gas out. I ended up replacing it with Dorman 692-137, which was good because even the lines were rusting out. Before I even attempted to put the new sending unit in, I tested it with a multi-meter and it worked as it should. I installed it into the tank reconnected everything and put the car back together. I put some gas in it and started the car (fuel pump engaged right away) and the fuel gauge went from past full to empty. Added some more gas, fuel gauge stays at empty. If I test the tank plug only, I get 10 ohms. If it run my multi-meter from the tank plug (purple wire) to the plug behind the back seat, i get 53 ohms. I cannot figure out what is going on.

 

I personally will connect the new Sending-Unit before installing it into the Fuel Tank.
Then I will turn ON the Ignition Switch, and move the Float-Arm of the Sending Unit by hand.
Having someone else watch the Fuel Gauge, make certain that the Sending Unit operates properly.

Note most Sending Units from this era will be Grounded via contact with the Fuel Tank.
In this situation, a wire is needed to contact the Body of the Sending Unit to a Ground Location on the Car.

You will have to see if the Variable Resistance of the Sending Unit, matches the specified range.
Take these measurements on the Sending Unit, and then at the Fuel Level Gauge Connection.
This can help rule-out a Wiring issue/ excessive resistance.

Hopefully you do not have an issue with the Gauge itself.

 

Figured out the issue, apparently the fuel tray is broken and was blocking the sending unit from moving. Should I replace the tank or just remove the plastic tray?

 

Since you asked, were this mine, I think I'd get a new tank VS trying to extract the broken baffle out of that one. Yes, I know the new tank cure is fraught with peril itself, (a search will reveal a bunch of threads about fitment issues with what's available nowadays for new tanks) , but once fitted it'd at least be in it's "as designed" state with the baffle. I can't say with 100% certainty, but I think I remember reading here somewhere that the baffle is to keep the pump from sucking air while making turns with half a tank or less of fuel, hopefully someone who knows for certain will pop in and let us know if my memory is correct or not on it.......

 

I like OrangeBird's recommendation; but I would go to a 4th-Gen Tank and Tank-Unit (Pump/ Sending-Unit).

And like I said, always test the Sending-Unit outside of the Tank first.