baconline: I am currently having the same problem

Siena: mxxpunkxx!, we're supposed to guess the make, model and year of your car, right?

mxxpunkxx!:
Just changed my fuel pump after it failed. Had to get the complete unit
I have gas at half-tank full, but right now, the fuel gauge on the dashboard is showing Zero (empty with the light on)
Had to call an auto-electrician, who brought it out and tested it by moving the floater up and down
It was registering on the dashboard, but when we fixed it back inside the tank and cover it, it wont register any reading.
OK, I topped with 20 litres of gas just to be sure, the same thing.
But when we bring it outside the fuel tank and test it, it works perfectly.
Just confused.

ZIM DRILL: ask your auto electrican to do this put back the old pump and see if it gauges reads fine if it does it tells you twon things 1 the wires from the pump connection to the dashboard gauge are fine 2 the new pump fuel gauge is faulty

Siena:

...The gauge reads fine when the float arm is lifted and released manually, when the unit is out of the tank.

Siena:

From what the OP's posted, the new fuel delivery unit's fine. The gauge reads fine when the float arm is lifted and released manually, when the unit is out of the tank. So it's a float fouling issue.