A little back ground. I have a '98 4Runner 5vz, auto, 4x4. I am turbo utilizing the FIC6 for tuning. So far everything has been going well, no major issues. I had a thought and was curious if anybody here has had experience with this. The ECM is always looking for a "close to zero" fuel trim which is why in the FIC6 you have to adjust the o2 map AND the fuel map to try to get that close to zero trim while richening up in boost, but... What if I were to get the URD O2 simulator (I would be deleting the cat). The simulator tricks the O2 into thinking the cat is fine, thus fuel trims are good... So, Would I be able to install the O2 simulator then be able to only adjust the Fuel table in the FIC? Since again, fuel trims will be tricked into always being good.

Essentially I feel like it would be easier to dial in my tune if I didn't have to use the O2 table.

What do you guys think?

You've mistaken what the o2 simulator does. It just allows you to remove the cat and not get a CEL for not having a cat. You will still build your tune using your front o2 in closed loop. I've heard some people just remove the front o2 and make the entire tune open loop. Then you wouldn't have to tune around a functioning o2. I wouldn't recommend that, but it's been done.

But how does the simulator eliminate the CEL? Isn't it simulating a "good" LTFT to eliminate or not produce the CEL?

The O2 simulator only allows you to bypass the rear o2- which only monitors cataylist efficiency.

Correct, but what is it actually monitoring? How does the sensor know whether the cat is good or not?

It's monitoring the presence unburnt oxygen still in the exhaust post-cat. If oxygen still exists it often will combine with carbon and create carbon dioxide (think greenhouse gases) or worse, carbon monoxide. I found out in my testing and by replacing the rear sensor, it does have a very small affect on open loop fuel trims but does diddly-squat for closed loop.

So when you say monitoring, you mean monitoring LTFT? whether the post cat exhaust is rich or lean? Correct? A stoich O2 sensor should read around .45v, .1v being lean, .9v being rich. In a sense the simulator is simulating a constant .45v "tricking" the ECM into seeing a "good" cat even if it is bad. Correct?

Sort of. The rear O2 has a much wider variance before the ECU will actually start adjusting STFT's or LTFT's. It sends the same signal for the most part but how the ECU uses it is much different. It relies almost exclusively on the front sensor. Now I'm not saying you can't simulate the front sensor, but it will be more work that just giving it a flat .45V or something. Without the voltage curve of too lean, too rich, too lean, too rich that these older sensors send, the ECU will just start to go too lean or too rich and not correct it. Would become a bit of a tuning nighmare unless you can send it a constant wave signal to keep fuel trims in a tight range.

You can however, pull the front O2 sensor completely. It throws a CEL but it will just stick to the stock fuel map that's loaded on the ECU from the factory and then you can tune it however you want from there. Does cause it to have bad gas mileage and foul plugs and run rich though.