O2 sensor mystery

[spy1309]

93 Geo Metro 1l, 5 speed manual.
Tryng to solve the shaky idle problem I disconected the O2 sensor to my surprise the idle evened a litlle bit better than before, I pluged it back the idle again becomes shaky and at the end of the tail pipe you can hear puf puf sounds, unpluged it again idle evens up and no puff sounds at the tail pipe.
The check engine light never came on during and after the disconection of the O2 sensor, nor any codes were set.
What gives I dont understand.
And second question is normal for the idle to drop when the cooling fan kicks in? doesn t supposed the ECU to compensate for the electrical load?
any opinions greatly appreciated.