Can anyone tell me why with a stock d16y7 w/ a y8 manifold and a full aftermarket exhaust is giving me better fuel economy without 1 o2 sensor, and it doesn't seem to matter which one is malfunctioning or missing? Both my car and my mom's seem to get better fuel economy if the engine is running in open loop, I always thought it would have adverse affects on fuel economy, but apparently this is not the issue at all, and otherwise I notice barely any difference in throttle response or engine function between either closed or open loop, can anyone shed some light on all of this?

 

Only the upsteam sensor controls fueling. The downstream is purely for monitoring the health of the catalytic converter. As you have found unplugging either puts you into open loop.

My suspicion is that your upsteam sensors are old and are either lazy or biased creating a false lean reading. This will cause the ECU to add to the short and long term fuel trims reducing fuel economy vs the open loop maps. This condition won't set a check engine light until you run out of adjustment with the fuel trims and then you get a check engine light for lean bank 1.

To know for sure a full on scan tool would be helpful. Also tossing a high quality upstream sensor in would confirm. OEM or NTK sensors. I haven't personally used a Denso sensor but that should also work well.

The other possible cause is an exhaust leak upstream of the sensor creating a lean condition. Or a misfire would create a false lean condition. Again a proper scan tool would really help diagnose what is happening.

 

Well said sir.