I just had my fuel pump replaced and now my fuel gauge is way off. I can't tell when I'm on empty, I know it's somewhere between the half tank mark and full, but I don't know. I need to know how to fix the gauge.

If the trouble with the gauge started after you worked on it then you likely damaged or created a restriction to the movement of the sending unit. You will have to remove the tank and double-check your work.

Just replaced fuel pump and fuel indicator sensor on my Bonny and fixed the problem,this seems to be a very common problems with this car

9 times out of 10 the fuel pump/level sender assembly is the cause. Sometimes the gauge pod is faulty, and there have been cases where there were either poor connector pin fit issues and/or chaffed/melted/cut wires in the harness that leads back to the fuel pump. It is an extremely common fault and would be surprised if the pump/level sender assembly wasn't the cause.

I hope this info helps!
~subyguru~

I've never had to replace one, but I do know that it is somewhere in the guage cluster. Although, chevy trucks have always had a problem with the fuel guage "floating" up and down.

If the sending unit was connected and grounded, I would guess the gauge in the cluster is faulty.

You have blown a fuse..check fuse box. There may be one fuse box one in the cabin but also check for the fusebox in the engine compartment.

You will need to replace the fuel level sensor, this involves the removal of the fuel tank to access the sending unit.

The wire between the dash and the fuel tank is connected to ground somewhere. there is a control that fails too but if the control failed the temperture gauge also won't work. The wire can get in troble inside the tank too as if someone tried to steal the gas with a hose. if you can disconnect the connector ( wireing ) at the fuel tank and turn the key on and the gauge goes to empty you know the troble is inside the tank and the pump sender unit will need to be removed from the tank to correct the problem or even the gauge unit replaced.

Trouble code P1627 means: PCM A/D Conversion Malfunction

This is a fuel sending unit code and it directly related to you goofy gas gauge. This is a common problem in 2nd Gen Isuzus. The fuel sending unit needs to be replaced. Most places will tell you that you must replace the entire fuel pump assembly module but you can buy just the sending unit.

Other faults to refer this problem and other codes: P0141, P0300, P0404, P1404, P1627, with this synthoms:
hesitates sometimes on takeoff, gas gauge gives random readings(sometimes will read empty, or full, or somewhere in between), Fuel light flashes sometimes.

Hope helps.

in side the tank on the side of the pump there is what thy call a sender not to be confused with the sending unit. The sender is atached to the float and tells the gauge how much fuel you have.