The fuel level sending unit is bad. It is part and parcel with the fuel pump in the tank and the whole module will need replaced (96 and after). Prior to 96 it will be an in tank unit that will separate from the pump and can be replaced by itself. Don't know about the carb'ed engine.
how do you fix a gas guage on a 1989 ford aerostar
Hate to be the bearer of bad news, but My 99 Suburban has had problems with the gas gauge bouncing unexplainably between E and F, and doesn't read correctly. Turns out the Fuel Pump and sending unit attached to it needs to be replaced (just died in my driveway today). It is a time-consuming job, as the fuel pump assembly is located inside the gas tank.
Then fix it
Have you checked the gas floater in the gas tank?
Your gauge could be bad but sometimes the floats in the gas tank get holes in them and fill with gas. this causes the float to stay at the bottom and your gauge to read empty. U might also check the wiring from the sender at the fuel tank to the gas gauge itself.
The fuel sensor is in the gas tank and probably needs replaced, the tank will have to come out to fix it.
Could be bad gauge or float stuck in fuel tank
Its probably the float in the gas tank that tells how much fuel is left in the tank... its sticking.
Most fuel gauge problems are the result of a faulty fuel gauge sending unit located in the fuel tank.
sending unit
check sending unit in tank and check gauge for proper operation and connections, and grounds
check the wiering /our your sending unit no good