04 Burb L59 PCM/Fuel Gauge Issue

[GMT800Dude90]

Ever since getting my 04 Burb, I've had an issue with the fuel gauge reading right. I bought a rebuilt cluster and it works as it should. But I still had a problem with it dropping out and coming back and never knew what my true fuel level was. 

 

I just replaced the old Delphi pump yesterday under warranty with a brand new one. Took it to the gas station and filled it up. Read dead empty with the fuel light and fuel range low. 

 

I picked up a used PCM at the junkyard from another 04 Burb and put it in, and it seemed to correct the issue, until it read dead empty on a full tank. I remember the PCM was from an LM7 Gas truck. Mine is the L59 Flex. The main part number was the same, so I figured it would be fine. 

 

We drove it on a 160 mile trip and the gauge moved like it should, just backwards. And when we got home it was on 1/4 tank, the actual fuel level was 3/4. So I thought that the used PCM from the gas truck would be the issue since the connector on the gas pump is the wide rectangle, and mine is the small square one. So the wiring was backwards. 

 

I got up this morning and put my original PCM in, and the gauge went to half. True level is 3/4...it wasn't acting wonky so the new level sender is good. But I'm worried that running the VIN T PCM messed something up, since the pump and injectors are different in my L59 VIN Z truck. 

 

Can someone confirm if it would have hurt something using the different PCM, even though the part number was the same? I suspect my original PCM is bad on the fuel level circuit since it won't read the correct level. Current true fuel level is just under 3/4 tank, but it's reading half. 

 

Any help would be truly and greatly appreciated. I hope I was as clear as I could be. It's hard for me to explain it through text. Thank you.