TBI wont go to closed loop

[oltronj]

91 Chevy V3500 454 with a 4L80E and a 205 tcase, trucks seems to run good but noticed the gas mileage has dropped (went from not so good to bad), started poking around and put a 3 wire O2 sensor in it just to see if that would help. I learned of the trick to jumper the 2 terminals on the diagnostic port and start the truck to get the check engine light to flash to tell if it is in closed loop or open loop and sure as heck it was just staying in open loop all the time. So what all will keep the computer from going into closed loop? I tested the coolant temp sensor that is by the tstat housing and it checked out ok at all the temps I checked it at. I am gonna test the O2 sensor and see what it is doing. I have read that the Bosch O2 sensors can give these trucks fits and I dont remember what brand came with the 3 wire conversion kit so if it looks suspect it will get a new AC Delco one. Any other suggestions on what to check or information on this would be appreciated.