First off, I have searched and read read read but I can not find the answer maybe because to many brain cells lay back in the 70's and 80's.

My pickup 1998 GMC 1\2ton 5.7 \ 4L80E Bone stock.
History: the pickup was delivered with a 305 / 4L60E which I swaped to the 350 I used the 350 harness and censors, the doner equipment was out of a 98 3\4 ton the PCM has a wheatley tune mostly to use the 4L80E.

Problem: Lately it has started running rough at idle (not like a miss but enough to feel in the body, brake feels normal) the fuel mileage has dropped off from 16-17 to 12-13.
the engine was rebuilt (bone stock) attached is an open office file on every drop of fuel the engine has burned from the swap.

Please change the file extension from .pdf to .ods open office can be downloaded at openoffice dot org
Microsoft excel file change Pickup.pdf to Pickup.xls

I have the battery disconnected to see if that will do anything besides make me reprogram the radio

Anything I missed please let me know.

 

Here is a scan from my Autel MS509 Scan tool.

 

Whats the fuel pressure read? Also, clean your MAF and check for a vacuum leak.