mgrotel

iTrader: (3)

I have always been under the assumption that the rear O2 sensors ONLY monitor whether the catalytic converters are functioning properly and I have heard other people verify that. I have also heard some others that say the rear O2 sensors actually have engine input, specifically with determining the a/f ratio of the banks. I always thought that was the job of the pre cat O2 sensors. Can someone clear this up for me? thanks

djamps

iTrader: (1)

I've spoke of this in many threads, long story short (please search) -- on my model year (04.5) with stock widebands, the lower o2's are verified to be used in closed loop. At times, overriding the wideband input as verified in my cipher logs. In the FSM, it never states that the lower o2's are not used in fuel trims... only that they can be used in certain conditions without going into any detail about what defines those conditions. In my case, those conditions were closed loop sustained light throttle for more than a few minutes (e.g. freeway cruising).

i spoke with an infiniti master tech (who just finished school so things are still 'fresh' in his mind) and he confirmed my findings. There's some algorithm the ECU uses: certain speed, certain throttle position, and a certain amount of time passing where the ECU switches to the lower o2's until the load changes or otherwise conditions fall out of the criteria. Basically the ECU looks for things very 'steady state' like cruise control before utilizing the lower o2's.

This poses a problem for test pipes or HFC's that have o2 extenders. The ECU will always see 'lean' when it looks at the lower o2's. Eventually the long term trims start steering rich as well. Many of you notice this in greatly reduced MPG's and lean codes especially on longer drives. If you have an A/F gauge, you'll see alot of time spent at 12-13 instead of 14.7 when cruising. Try resetting your ECU/fuel trims... I bet you feel a difference when the overly rich trims are back to stock for the next few miles.

That said, none of this has any affect on WOT (e.g. dyno numbers or track times) because none of the o2 sensors are used in open loop.

mgrotel

iTrader: (3)

wow, interesting. thanks. how did you verify that? just curious

djamps

iTrader: (1)

This all came about when I was troubleshooting the exact symptom I described -- too rich sometimes, **** mpg's, ECU overcorrecting and throwing lean codes on both banks. After doing the usual and passing with flying colors (smoke test intake and exhaust), I took extended drives and logs with and without nonfoulers. As a self-tuner I'm extremely **** about my tune and this was driving me crazy. I was relieved to discover the 'problem' was really so simple.

As you can guess, the non-foulers stayed off my car and never had a code or overfueling since. Went from <18mpg to 25+ on the freeway and the car is noticeably more peppy at part throttle.

mgrotel

iTrader: (3)

wow, very interesting. how big were your non foulers?
you think i should cut off my extensions on my test pipes to ensure the rear O2s get good readings?

djamps

iTrader: (1)

They were about and inch or two and had a small hole on the end. Enough to get the sensors out of the stream completely and let a little exhaust in.

It's completely up to you... I guess it depends on emission testing in your area. I MAY have to put them back on temporarily for the next emissions test if the deletion of the 04x0 codes causes a fail or if I can't drive long enough without throwing them. It usually took over a hundred miles until I throw a bank lean code with the nonfoulers, plenty of time to get thru emissions.

My suggesion -- get normal bungs welded on so you can go either way easily.

mgrotel

iTrader: (3)

my test pipes are gonna look like they got beat up, already have bungs welded up top for my wideband, now two more next to the extensions, lol

djamps

iTrader: (1)

I got 4 bungs on each of my downpipes...no such thing as too many!

mgrotel

iTrader: (3)

lol

davidv

iTrader: (2)

From the service manual:

The heated oxygen sensor 2, after three way catalyst 1, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of the heated oxygen sensor 1 are shifted, the air-fuel ratio is controlled to stoichiometric, by the signal from the heated oxygen sensor 2. This sensor is made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. Under normal conditions the heated oxygen sensor 2 is not used for engine control operation.

djamps

iTrader: (1)

Thanks david.

And if you modify your tune/intake/exhaust in any way, you can throw 'normal conditions' out the window and assume sensor 2 is working overtime. The way I interpret the wording in the FSM, it is unless certain conditions are met, sensor 2 is doing most of the work...

mgrotel

iTrader: (3)

the way im reading it, it sounds like O2 #2 only adjusts to stoich, anything else would be from the O2 #1. correct?

djamps

iTrader: (1)

Correct... stoich = 14.7 which is what you will be targeting for anything less than 50% throttle on an N/A tune.

mgrotel

iTrader: (3)

ok, so anything done on a dyno tune would not involve the rear O2s, but everyday driving would be much controlled by the rear O2s. djamps, do you recommend that i road tune my car myself for daily driving after i am done getting a professional tune for max power? do you just mess with the timing since part throttle will always go to stoich?
thanks

davidv

iTrader: (2)

Correct. If you alter intake or exhaust these are NOT normal conditions. Now sensor #2 is part of the engine control operation. If you do ghetto-mods such as Brillo pads inside the bung, these are NOT normal conditions.

davidv

iTrader: (2)

Sensor #1 is the most important sensor. It detects the amount of oxygen in the exhaust gas compared to the outside air. Sensor #1 signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio.

mgrotel

iTrader: (3)

^ but only on hard throttle input? since the #2 O2 adjusts to stoich, which is an a/f?

R350Zz33

iTrader: (4)

i have a crazy idea get uprev then you dont have to have 2 bungs

mgrotel

iTrader: (3)

I have the tuner version and a fully built motor on spray

Not to mention the rear O2s still take readings even when cat codes are removed.

djamps

iTrader: (1)

Sensor #1 widebands (on 04+ models) are much faster and have more resolution than #2 narrowbands. So during more aggressive/dynamic driving the ECU will prefer sensor #1. In steady state (freeway cruising) the ECU will switch to #2 after some time. This is what I noticed in my logs...

Makes sense nissan would only use #2 narrowbands for targeting stoich and long term trims... they aren't fast enough for targeting arbitrary A/F's in dynamic loads.