car: 86
motor: late bigport tvis with cams, stock ae92 ecu with tvis (early ae92) stock rwd extractors

issue: im aware that fitting an 02 sensor should only improve mileage at cruising speeds however since i have fitted one it has altered my idle speed, my car has always needed to idle at around 1100-1200rpm im assuming because of my unknown cam's, but when the o2 sensor is plugged in it sits around 1400-1500rpm and gradually creeps up, not like idle hunting bouncing up and down but slowly at the lights, and then drops back to 1200 then climbs again.
when i disconnect it goes back to normal 1100 and sits there.
02 sensor is a second hand one that looks ok. i figured even a worn out one would be the same as not running one at all.
cant check my mileage at the moment because my fuel sensor is playing up but i could get 440km's out of my tank before and when i went to fill up could only get 39L back in it.
i know i could just piss it off but its my daily and any saving on fuel would be a plus. car does not smell too rich and still has great performance