[mwr]

My city MPG can drop to 10mpg, but often averages 12. Highway is 19-20, which is normal. Does any of you FX45/50 owners get better gas mileage around town?

I have no check engine lights "on" and other than 115,000 miles on the OEM O2 Sensors, I'm considering the possibility of replacing my 2 "upstream" O2 sensors. I'm also thinking O2 Sensors are the next step to improving city mpg if I have already performed a Seafoam treatment and cleaned my MAF sensor. So what do you think? And why do so many sensor vary in price?

I'm also concerned these cheap O2 sensors on Amazon may prematurely fail so I want to stick with Denso, Bosch or NTK/NGK, but if anyone has used the Walker or other Ebay vendor brands, please let me know if you are satisfied and if your mpg or performance improved after you replaced your sensors.

At O'reilly's these name brand sensors run $80-$120 each and since there are 2-upstream and 2-downstream sensors that's a lot of clams. So I'm thinking of only replacing the upstream if I am trying to improve my city mpg vs. trying to pass an emissions test. ...And remember I don't have any sensor codes flashed on my OBD2 Scanner.

So before I start throwing more money at this problem I'm hoping there are some testimonials out there to guide my decision. And I full accept most mechanics will say to do nothing until you get a TDC. However, I also know all O2 sensors degrade over time for a lot of reasons. And frankly, it's easier to just replace them vs. test them.

Does anyone know what the OEM brand of sensors Infiniti uses?

Here's a chart I created with extra information and part numbers I have be compiling for your library.

 

[mwr]

In another Infiniti Forum, one professional mechanic comments he has only seen "bad results" from using Bosch O2 sensors and recommends Denso or to use the same OEM type that came with your car.

So I did some more research and and found a number of forum threads that have a lot to do with how the ECM sensors communicate with several sensors to determine the most optimal fuel trim. Below is a summary of several easy to do procedures to keep your 2003-2012 FX45/FX35 operating at peak performance. You can also print out the attached pdf file.

2003-12 FX45/35 ECM IDLE AIR RELEARN
+ THROTTLE RELEARN + MAF RELEARN NOTES (with edits)
("2003-2012 Infiniti FX35 - FX45)

​

Before performing Idle Air Volume Learning Procedure, make sure that all of the following conditions are satisfied. Learning will be cancelled if any of the following conditions are missed for even a moment.

The first 2 procedures are done with a cold engine and only take 2 minutes. (EASY-PEASY!) The next 2 procedures are done with a hot/normal engine and are also easy to do if you have a OBD2 Scanner that lets you reset your Trouble Detection Codes (TDC). Here’s and Amazon link to $23 Scanner I used:
https://www.amazon.com/gp/product/B07YHDFSP3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Perform Accelerator Pedal Released Position Learning (Cold Engine)
Note: This must be performed each time harness connector of accelerator pedal position sensor or Engine Control Module (ECM) is disconnected.
2003-2012 Infiniti FX35 - FX45 Accelerator Pedal Released Position Learning Procedure | Infinitihelp.com

1) Make sure that accelerator pedal is fully released.
2) Turn ignition switch ON and wait at least 2 seconds.
3) Turn ignition switch OFF and wait at least 10 seconds.
4) Turn ignition switch ON and wait at least 2 seconds.
5) Turn ignition switch OFF and wait at least 10 seconds.

Perform Throttle Valve Closed Position Learning (Cold Engine)
2003-2012 Infiniti FX35 - FX45 Throttle Valve Closed Position Learning Procedure | Infinitihelp.com
1) Make sure that accelerator pedal is fully released.
2) Turn ignition switch ON, but do not start engine.
3) Turn ignition switch OFF and wait at least 10 seconds
==> Confirm the throttle valve moves during above 10 seconds by
confirming the operating sound. If not then proceed...

4. Turn ignition switch “ON” (without starting vehicle) and wait at least 2 seconds.

5. Turn ignition switch “OFF” wait at least 10 seconds.

END


Idle Air Volume Relearn Procedure To Reset ECM (Hot Engine)
Complaint: I cleaned my throttle body last night and now my truck idles a tad higher. Also the C.E.L. is on.
RESPONSE: This problem is perfectly normal after cleaning/replacing the throttle body. To fix it just perform this Idle Air Volume Relearn Procedure.

2003-2012 Infiniti FX35 - FX45 2003-2012 Infiniti FX35 - FX45 Idle Air Volume Learning Procedure | Infinitihelp.com

1) Start engine and warm it up to normal operating temperatures by driving the car for 10-15 minutes.
2) Check that all items listed below are in good order:
* Battery voltage: With engine running, alternator is putting-out +12.9V- 14V.
* Engine coolant temperature: 70 - 100 C (158 - 212 F)
* Park/neutral position (PNP) switch: ON (Shifter in Park or Neutral)
* Electric load switches “OFF.” Ex: Air conditioner, headlamp, rear window defogger, seat heaters.
* Steering wheel: Neutral (Straight-ahead position)
* Vehicle Cruise: OFF/Stopped
3) Turn ignition switch OFF and wait at least 10 seconds.
4) Confirm that accelerator pedal is fully released, turn ignition switch ON and wait 3 seconds.
5) Quickly depress the accelerator pedal 5 times within 5 seconds. (Aka “Stomp Test.”)
6) Wait 7-10 seconds, fully depress the accelerator pedal… and keep it depressed for approx. 20 seconds until the MIL (Check Engine Light) stops blinking and turnes ON.
7) Fully release the accelerator pedal within 3 seconds after the MIL (aka CEL) turned ON.
8) Start engine and let it idle.
8) Wait 20 seconds.
Here’s a chart to describe the above steps in strict secquence.

9) Rev up the engine two or three times and make sure that idle speed and ignition timing are within the specifications. Note: Your idle speed = 650 ± 50 rpm (in P or N position)
END

NOTES: If the above 8 steps do not turn on the CEL as described, try this sequence commonly used on Nissan Pathfinders to accomplished the desired results:

----- EASY METHOD TO RESET ECU ON NISSAN 2005 PATHFINER -----

1. Ignition "ON", count to 3 (1000-1, 1000-2, 1000-3)

2. Depress/Release pedal 5 times...hard and fast within 5 seconds.

3. Count to 10 (not 7!) and fully depress pedal

4. Count to 12 (WIF Light should be blinking and then will turn “solid-on.” When it does turn “solid-on” release pedal.

5. Count to 10, depress pedal

6. Count to 10, release pedal.

7. Turn Key off.
END.

NEXT PROCEDURE… And for added measure do this to reset how the Mass Air Flow (MAF) sensor communicates with the ECM.

MAF Sensor / Fuel Trim Relearn Procedure (Hot engine)

2003-2012 Infiniti FX Clear Mixture Ratio Self-Learning Control Data | Infinitihelp.com

1) Start engine and warm it up to normal operating temperature.
2) Turn ignition switch OFF.
3) Disconnect mass air flow sensor harness connector, and restart and run engine for at least 3 seconds at idle speed.
4) Stop engine and reconnect mass air flow sensor harness connector.
5) Make sure Detected Trouble Code (DTC) P0102 is displayed.
6) Erase the DTC memory.
7) Make sure no codes are stored in the ECM.
8) Run engine for at least 10 minutes at idle speed.
END.

 

[mwr]

With my $23 OBD2 Scanner I was able to perform the MAF sensor relearn steps above and now my idle is holding right at 650RPM. (Before the engine would idle high in the the morning and then average 750RPM after the engine heated-up.)

The OBD2 scan tool also reveled the information below to me... and based on this data (at idle) I don't plan on replacing my O2 Sensors unless someone says I need to replace them? Note: I'm out of school on this subject so I would appreciate more discussion on this subject. However, at idle I think if the voltage remains at all times below 1V then the sensor is okay. Is this true or is it more likely I have to increase the RPM to ~1,800 and then take measurements?

UPSTREAM SENSORS AT IDLE

B1S1 Voltage would oscillate between 0.07V - 0.73V (occasional 0.90V) and held relatively steady at 3.9%-4.7%
B2S1 Voltage would oscillate between 0.04V - 0.45V and held relatively steady at 3.1%-4.7%

DOWNSTREAM SENSORS AT IDLE/b]

B2S1 would oscillate between 0.06V - 0.18V and held relatively steady at 3.1%-4.7%, with SHRTFT= 99.2%
B2S2 would oscillate between 0.45V - 0.46V (is this okay?) and held relatively steady at 3.1%-4.7%, with SHRTFT = 99.2%

Notes: I'm not getting any TDCs. My complaint is that my city MPG seem very low at 10MPG. However, after performing the above relearn steps, I think my dash data computer that measures MPG is reading in the 14-16 City-MPG so long as it is not stop-and-go traffic. So maybe I'm seeing some improvement. It's too soon to tell.

Conclusions: From these numbers at idle I believe my ole O2 Sensors are still operating normal. So I will not be replacing them. And if I can confirm my city MPG is better, by virtue of reading the onboard MPG computer, I will let you know.

 

[mwr]

OP UPDATE: After performing all of the engine "relearn" steps which I hoped would improve my city MPG... it did not! And I am still getting 8-10MPG in stop-and-go traffic. Is this normal?

I get that if you are just waiting at a traffic light... that your MPG will be crappy.... because you are not moving... but I'm still surprised a rich running engine would "draw-down" my overall MPG numbers this much.

So then... other than replacing my 2 "upstream" O2 sensors "on a lark," I have no other ideas on how to improve my city-MPG. So what am I missing?

Note: I am aware, in general, O2 sensors wear-down over 100,000 miles and my car has 117,000 miles and my engine is not flashing any TDC. So that begs a question when my city-MPG is low: Are my "upstream" O2 sensors good or not?

Note: I am still getting 19-20 MPG on the highway, which is good, However, I'm still having trouble accepting this V-8 has normal fuel trims when I'm only getting 10.7 MPG overall. ...And yes, 80% of my driving is around town.

Maybe the question I should ask is this: What is your FX45 city and hwy MPG?