Air to fuel ratio problem?

Hello, what are you referring to when you say 16.1 17.1? Is your scan tool giving you an air fuel ratio number? And you're getting an oxygen sensor code? What code are you getting?

Okay. So, now you have an Acura engine in it, it looks like all the 2000 Integra's have a 1.8-liter dual overhead cam, is it a VTEC engine? I just want to make sure I'm looking at the correct wiring diagrams. And was any work done to the engine before it was installed?
It's possible you're still getting the rear oxygen sensor code because the oxygen sensor heater circuit is not working. The 1st diagram shows the rear oxygen sensor wiring, the heater circuit will be the 2 wires at the sensor that are the same color. There should be 12volts going across those 2 wires. In the 2nd picture is where the wiring harness for that sensor runs up into the body and there is supposed to be a rubber grommet there to prevent the wires from rubbing through the insolation and shorting out, it was common on these for people to leave the grommet either out or push it up. If the wires are making contact on the body at that point, normal vibrations cause the wires to end up being cut or shorting out on the body (frame) of the vehicle. Ill post the fuse also that powers the heater circuit, But the sensor will not work without the heater circuit functioning. The engine computer might also monitor that circuit to verify it is working by monitoring either current flow or voltage. So, check that the wiring at that grommet is not shorted out.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Okay, look up at my last post, refresh this page. I put the wiring diagram and what to check on the rear oxygen sensor. Here is some additional information. Once you get the rear Oxygen sensor issue taken care of, and if you still have an issue with a lean condition, you will most likely get a lean code. I'm not sure how accurate the air fuel ratio gauges are. Scan tool live data would give you a much better idea of what the engine computer is doing with its fuel strategy.

Does the vehicle have a turbo? Sounds like the vehicle the engine came out of had a lot of modifications and the gauges for all of it are still hooked up and functioning. ATF type D is a high-grade automatic transmission fluid. So maybe that's what he was referring to.
It's difficult to tell with vehicles that have been modified so much and then taken apart. Who knows what's left and what's not.