There seems to be no table to get it in open loop. I had the same thing on my 1998 S10 with the 2.2 4cylinder. It had a table to turn long term trims off by maxing out the temp table. But it would still use the short term trims and the o2 sensor would still operate correctly. So I unplugged the o2 sensor and then it went open loop. Try unplugging the o2 sensors.