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Rich air fuel ratio. Need help!

Hello Everyone! I have a Nissan 370z which I had tuned at z1 motorsports about 8 months ago. I had a CEL from one of my cats (p0138) and wanted

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Old 11-17-2015, 10:59 AM   #1 (permalink)
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Default Rich air fuel ratio. Need help!

Hello Everyone! I have a Nissan 370z which I had tuned at z1 motorsports about 8 months ago. I had a CEL from one of my cats (p0138) and wanted it gone as I thought this code was just from my high flow cats. Jon (at z1) was kind enough to let me use his cable to delete the code and after doing so I decided to toy around with the cable and monitor ecu parameters such as air fuel ratio in both banks etc. I logged some data that it wont let me post to this thread (says invalid file). Anyways the A/F ratio in bank 2 during closed loop idle is always around 14.1-4(Ecu parameters- INJ pulse width is always 2.6ms at idle and A/F WB sensor is 1.8volts) and bank 1 is always around 14.6-8( INJ pulse width is 2.5ms and A/F WB is 2.2volts).

Pulled from FSM. (factory service manual.)
"The mixture ratio feedback system provides the best air-fuel mixture ratio for driveability and emission control.
The three way catalyst 1 can better reduce CO, HC and NOx
emissions. This system uses A/F sensor 1 in the
exhaust manifold to monitor whether the engine operation is
rich or lean. The ECM adjusts the injection pulse
width according to the sensor voltage signal. For
more information about A/F sensor 1, refer to
EC-30, "
Air Fuel Ratio Sensor 1
. This maintains the mixture ratio within the r
ange of stoichiometric (ideal air-fuel mixture).
This stage is referred to as the closed loop control condition.
Heated oxygen sensor 2 is located downstream of the three way catalyst 1. Even if the switching characteris-
tics of A/F sensor 1 shift, the air-fuel ratio is controlled to stoichiometric by the signal from heated oxygen sen-
sor 2

As i had stated before I threw a p0138 code and my question is basically interpreting the bolded section above. Is it saying that air/fuel ratio is controlled by the downstream sensor or not? and if it is not does anyone have nay suggestions in solving this rich condition at idle? p0138 code is "Ho2S 2 signal is less active than signal produced by front oxygen sensor. This code sets when HO2 Sensor voltage is greater than 999 mV for more than 2 minutes (time depends on model. Could be as high as 4 minutes)"

Now i have another question..
At WOT my air fuel ratio will go as rich as 12.1 and as lean as 12.8-9 throughout rpm, 3000-8000rpm. Is this normal or is it supposed to be consistent the whole way throughout?

Thanks for any replies.. ive has Jons cable longer than i probably should have because ive been trying to solve this issue for awhile now with no luck.
Ive checked for intake leaks( wouldn't a intake leak cause a lean condition instead of a rich one?) and exhaust leaks before the sensors.

edit- Start engine and warm it up to normal operating temperature.
2Turn ignition switch OFF.
3Disconnect mass air flow sensor harness connector, and restart and run engine for at least 3 seconds at idle speed.
4Stop engine and reconnect mass air flow sensor harness connector.
5Make sure Detected Trouble Code (DTC) P0102 is displayed.
6Erase the DTC memory. (This could be done by disconnecting the car battery for 30-45 min or with a scanner)
7Make sure no codes are stored in the ECM.
8Run engine for at least 10 minutes at idle speed.

followed these instructions and both banks air fuel ratio was at 14.7 at idle with no adjustments. After about 5 miles bank 2's air fuel ratio was at 14.3 and the adjustments were .342. Another thing i noticed is bank 2's vvel position sensor signal is always lower than bank 1's. Anyone ran into a issue with a/f ratio like this? any help appreciated.

Last edited by Wilson2608; 11-17-2015 at 02:41 PM. Reason: more info
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