So let’s say for some reason that my o2 sensor is unplugged. If I jump 0.2 or 0.3 volts to the harness going back to the ecu would it work into tricking the engine to run a little less rich to save some gas? I heard it needs to fluctuate between a set point in order for the ecu to not register it as bad (95 Acura legend). So what’s the verdict? Any ideas on how to make it fluxiate if I can’t set a straight voltage?
This is a bad idea.
There's a very good reason why ECU's default to running on the rich side when they are unable to get a valid reading from the O2 sensor and that is because running an engine too lean can cause quite substantial amounts of damage.
If you were able to (somehow) simulate the fluctuating readings within the "acceptable" range the ECU expects there is nothing tying those fluctuations to the actual fluctuations in airflow that is going through - so your simulated readings could be telling the ECU that it was at the lower end when the actual flow was at the opposite end of the scale - this would give a (relatively) large discrepancy between fiction and reality. And that could mean very bad, very expensive things happen to the engine.
Did I mention that this is a bad idea? Don't do it!
It's much cheaper to fix an O2 sensor issue than the consequences of unsuccessfully trying to spoof it, and you will be unsuccessful. Don't do it.
If you run an engine too lean it can cause detonation in cylinders, which is the uneven explosion of the fuel-air mixture. Detonation sends shock waves through your cylinder, piston rings, head gasket, valves, etc and can cause failure of one or more of those components. Running lean can also cause your engine to overheat as there's no excess fuel for evaporative cooling. At the very best you will get poor performance.
An O2 sensor replacement is generally an inexpensive part and reasonable on labor, often it is something a competent home mechanic can do. A working sensor will give you the best fuel milage and performance out of the engine, and will pay for itself in fuel savings over time.