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Old 09-16-2013, 10:46 PM
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Possible problems related to Code P0171

Hello everyone, I have been trying to research the possible cause of a code that just popped up on my WJ 4.7 (1999). The code is P0171 or fuel mixture too lean bank 1.

The first I noticed was Saturday when I started it took two tries and rough idle for a minute or so then all normal for a 50 mile trip. Next two times hard to start then 'check engine light comes on. Today same story, but seems like its not getting enough fuel.

The more you search, the more possibilities there are for the problem. ...seen O2 sensor, Map sensor, vacuum leak, fuel filter, pump, bad gas, air filter & more. My question for you more knowledgable owners is where would be the best place to start? I looked for the obvious vacuum leaks today.

I had a theory that six solid days of rain contaminated the fuel I just bought and finished off the filter/regulator. I was going to check the fuel pressure first. Any wise pointers would be awesome. It is not smoking or stalling- really drives fairly normal after it starts other than occasional slight hesitation and crappy mileage according to the VIC.

Thanks for reading !!


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Old 09-16-2013, 11:09 PM
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Re: Possible problems related to Code P0171

Crappy gas milage and the code P0171 sounds as a problem with O2 sensor.
I would not expect problems with the gas or fuel filter, otherwise the other bank should have the same problem. Does the exhaust smell raw gas, which would be an indication of too much fuel. Eventhough you got a fuel mixture too lean message, if the O2 sensor is faulty it could measure a lean mixture, while it is rich. Would explain the rough idle and higher then normal fuel consumption.
Do you have a scanner which shows live data? If not, and you have an Android phone, you could get the free app named Torque. You then only need a bluetooth dongle, which fits in the OBDII port and makes the connection between the android and the jeep.
You can then check the O2 sensor voltage. There are 4 O2 sensors, 2 upstream sensors (upstream of the mini cats) and 2 downstream sensors (so downstream of the mini cats). The upstream sensors are directly related to the fuel injection finetuning, while the downstreams are more for the mini cat performance.
Bank 1 upstream sensor is shown as 1-1, while the bank 2 upstream sensor is shown as 2-1 (the downstream sensors are the 1-2 and 2-2).
The voltage of the O2 sensor should go up and down in between 0.2V and 0.8 V and should do this change very often. If the reading is fixed or jumping a bit only, the O2 sensor needs to be changed.
Further to this, if the previous seems to be ok, you could check the fuel trim values calculated by the PCM.
These figures are shown as STFT and LTFT, which stands for Short Term Fuel Trim and Long Term Fuel Trim.
The STFT is the actual fuel trim , while the LTFT is an average over a longer period.
Let us know what you find.
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Old 09-17-2013, 12:05 AM
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That is the coolest app EVER. I just ordered the BT dongle so I can use it. Meanwhile, Im going to have a mechenic hook it up and see what he can figure out. I really hate to drive it like that. Thanks for your reply, and I will update the post when there is an answer.

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Old 09-17-2013, 12:07 AM
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PS , I'll tell him about the O2 sensors and fuel trim.

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Old 09-18-2013, 10:45 PM
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SOLVED! turned out to be a clogged fuel filter/regulator. Everything back to normal.
Thanks for the help.

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Old 09-18-2013, 11:04 PM
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Re: Possible problems related to Code P0171

Good to hear its solved, and even a cheap fix.
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