350 open loop
 

I have a 95 chevy g30 van, 350. This problem has been with me a couple of years, and I need HELP!
Gas mileage suddenly dropped. I did the usual, plugs, wires, O2, injectors (its TBI), etc. No change. Took it to a dealer, who said its stuck in open loop. I changed the coolant temp sensor, and even ran a ground wire to the sensor. Nothing.
Since it is OBD1, I have downloaded TTS Datamaster and got it running. I can verify that it STAYS in open loop. There is NO check engine light or codes thrown. It runs and drives pretty well, just LOW gas mileage (runs rich, I can smell it in exhaust).
Datamaster shows coolant temp of 190 degrees, so that seems ok. I just assumed that the O2 sensor wasn't getting hot enough, so I replaced it with a HEATED O2, and wired it up....same results, still stays in open loop.
I think I read that the 3 things needed to achieve closed loop were coolant temp, O2 activity (hot), and time (needs to be running x minutes).
What am I missing??? Can the ECM cause his and still be OK enough for the vehicle to make 1600 mile trips to California??
Help would be greatly appreciated!