Thanks for the clarification Jerry,
You have replaced the tank unit. And there was no change.
Reading Full all of the time means that there is an open circuit in that wire that goes from the tank sending unit to the PCM. Connector 1 pin 69.
The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor card. The position of the float arm indicates the fuel level. The fuel level sensor contains a variable resistor which changes resistance in correspondence with the amount of fuel in the fuel tank. The Powertrain Control Module (PCM) sends the fuel level information via the Class 2 circuit to the Instrument Panel (IP) cluster. This information is used for the IP fuel gauge and the low fuel warning indicator, if applicable. The PCM also monitors the fuel level input for various diagnostics.
Since the fuel gage is working some of the time and not working at other times, Then we have a poor connection somewhere in the wire from the Tank Unit to the PCM.
When the fuel gage goes all the way down to empty, that is an indication that there is a short in the Purple wire going from the tank unit to the PCM. Or the tank is getting low and the gage is beginning to work.
IF it is found that the wire from the tank unit to the PCM is fine and all connections are secure and no shorts to ground are found in this wire...then it is time to have the dash cluster checked for gage operation.
With the Tech ll scan tool we can actually sweep all of the gages and see if when they are commanded, how they react.
Since the communication between PCM and Instrument cluster to operate the fuel gage are done over serial data communications, you would see a ton more problems if that were an issue.
This is very important:
There are several connectors that may be causing your problem. C305, C203,C103 all carry the purple wire to the PCM for fuel level. These connectors need to be checked.
Back to another basic test, If you pull the connector off of the fuel tank unit and the gage should go up to or past full mark on the gage. If you ground the purple gage wire, the gage should read Empty on the dash.
If everything from the tank unit to the PCM is good.
Then you will need to have the cluster checked out.
I hope this information will aid in your locating the problem with your fuel gage.