P0171 ON 99 WJ 4.7L 185kmi - JeepForum.com
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post #1 of 12 Old 10-11-2020, 12:44 PM Thread Starter
crusher8101
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P0171 ON 99 WJ 4.7L 185kmi

My WJ sat for a week. When I went to start it, very rough starting, uneven idle until it reached operating temperature, idle was smooth at 6k, but accelerating was rough with sputtering and backfires.
CEL code P0171 came up. I donít have a scanner. I replaced the TPS and cleaned the IAC. Starts easier, but same symptoms and same code.
Any suggestions?
Any advice about buying O2 or IAC sensors on eBay?
Would really appreciate your advice!


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post #2 of 12 Old 10-11-2020, 02:47 PM
Wabatuckian
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Check for vacuum leak.
Check fuel pressure.
Check for proper injector operation.

Don't shotgun it. When you cold start, you're in open loop operation and your O2 sensors are providing no data to the PCM. Your IAC is operating because your Jeep is idling.

You're getting either too much air or not enough fuel. Good news is that you should have a schrader valve on your fuel line. (Mine, a 2004, does not, as a cost-cutting measure.)

I'd check for loose vacuum lines first.

Josh
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post #3 of 12 Old 10-11-2020, 02:57 PM Thread Starter
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Josh - Thanks! Iíll get busy with the vacuum lines right away.
Itís definitely running fuel-rich from the exhaust odor.


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post #4 of 12 Old 10-11-2020, 03:12 PM
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It's running rich??

Your code indicates a lean condition.

If the O2 sensor is sending a lean signal (high voltage) the PCM will attempt to compensate by enriching the mixture.

Do you have a scanner with live data? If not, Autozone should be able to use their scanner for it.

If you want to check to see if the O2 sensors are causing it, just unplug the front two. This will trip a couple codes, but it will force the PCM into open loop mode and deliver pre-programmed fuel trims.

Are you sure it's rich, though, or lean? Lean will burn your eyes.

Josh
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post #5 of 12 Old 10-11-2020, 04:21 PM Thread Starter
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P0171 ON 99 WJ 4.7L 185kmi

Well, the exhaust has a very strong smell of gasoline to it even after it has been driven to operating temperature. I assumed that meant a fuel-rich mixture.
Iíll also try your suggestion on O2 sensor test. No scanner but Autozone is nearby. I didnít know they did that. Cool.
Thanks!


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post #6 of 12 Old 10-11-2020, 06:20 PM
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See, that makes no sense to me, what it's doing:

1. If the O2 sensor is false lean, then it would make mixture richer.

2. The mixture is obviously rich, but the O2 sensor shows lean.

The only thing I'm not sure about is whether the long term fuel trims play a role in the open loop mixture. If it does, then that could explain the issue: A bad O2 sensor is showing lean, and the PCM enriches the mixture (short and long-term fuel trims) to compensate. If the LTFT data is then used as a baseline with which to provide the cold engine start mixture, it's quite possible you'll get this result.

FYI, if you keep running it rich, you'll burn out your catalytic converters, if that matters to you. I don't know if IL has emissions sniffer tests.

Josh
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post #7 of 12 Old 10-11-2020, 07:08 PM Thread Starter
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IL does have that test and Iím not eager to replace a Cat. I wonít be driving any distance until I can get to Autozone and get live data from their scanner.
Thanks much for your valuable information. Iíll update when I learn something definitive.


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post #8 of 12 Old 10-17-2020, 02:55 PM Thread Starter
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Had to replace 3 fuel injectors, clean the rest and replace the seals, replace the 7 yr old spark plugs, clean the O2 sensors, and did an oil change while I was at it. Runs like new now!


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post #9 of 12 Old 10-18-2020, 11:50 AM
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Very cool.

The I was wondering about the injectors, but didn't want to jump the gun.

Mine trims lean (about -14% on the LTFT) because the injectors need replaced.

Josh
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post #10 of 12 Old 10-19-2020, 12:02 PM Thread Starter
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Itís a time consuming job and the injectors are expensive if you donít want to clean them, but the performance difference is remarkable!


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post #11 of 12 Old 10-19-2020, 07:12 PM
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possibly bad MAF. start with cleaning it.
Get somebody to check live data to check MAF like TPS (slow increase, no steps)
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post #12 of 12 Old 10-20-2020, 08:27 AM
Wabatuckian
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No MAF sensor on the 4.7 WJ. Don't think there's one on the 4.0, for that matter.

Josh
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