Misfire codes, P0171, fouled plugs

Patient vehicle:
2000 XJ sport
107k miles
4.0
NP231
California exhaust

Symptoms:
Rough idle
Poor gas mileage
CEL P0300
P0301
P0303
P0171
Black soot spits out of tailpipe
Spark plugs from cylinders 1,2,3 are fouled. Plugs from 4,5,6 look new.

I can tell it’s running rich, I can smell gas. Gets about 6-8 miles/gallon. I’m going to run a dry/wet compression test tomorrow to make sure its nothing mechanical, but I’m leaning more towards a exhaust leak or a sensor somewhere. I had the third, non o2 sensored catalytic converter stolen back in 2012 and a straight pipe was placed between the precats and muffler with no ill effects. Passed smog for several years like this.

Any ideas to look for before I drop it off at shop? Thanks.




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I would lean towards a failed o2 sensor on bank 1 since the rear half seems ok. Another possibility is a vacuum leak (a large one) affecting only the front half of the engine.

A faulty O2 sensor is a logical suspect based upon the P0171, bad mpg's, and soot. What spark plugs are installed ? Plugs should be basic Champion or NGK, or Autolite APP-985 when distributor-less coil rail equipped.

Plugs are the basic champions. Here are the plugs and soot coming from tailpipe.


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On the California emissions, which o2 sensors are responsible for bank 1?


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Forward precat o2.

Jjcord2015 said:

On the California emissions, which o2 sensors are responsible for bank 1?


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Click to expand...

The 2 O2 sensors in the bank 1 portion of the exhaust -- should be easy to see -- it's the pre-cat closest to the front end

One sensor in front of the bank 1 pre-cat one downstream of the bank 1 pre-cat

what I am not sure of is which O2 sensor (upstream or downstream) is responsible for sending extra fuel to the 1st 3 cylinders

The ones with the 2 precats should have a total of 3 o2 sensors. The bank is split for more precise fuel control unlike the earlier models.

Heavyopp said:

The 2 O2 sensors in the bank 1 portion of the exhaust -- should be easy to see -- it's the pre-cat closest to the front end



One sensor in front of the bank 1 pre-cat one downstream of the bank 1 pre-cat



what I am not sure of is which O2 sensor (upstream or downstream) is responsible for sending extra fuel to the 1st 3 cylinders

Click to expand...

I’m just going to replace both on that precat, rock auto has them both for about $60. Idk if they’re original or not so I’ll just do it.


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trippled said:

The ones with the 2 precats should have a total of 3 o2 sensors. The bank is split for more precise fuel control unlike the earlier models.

Click to expand...

There are actually 4 total sensors on the Cali emissions. Two on each precat. There is a third catalytic convertor visible from underneath the vehicle, but I suspect that was put there so anyone looking would not say “hey you have no catalytic convertor!” Mine third one was stolen and I had straight pipe welded up in its place and had zero issues. No CEL and passed smog for years, as there are no o2 sensors monitoring this cat. Not sure it even does much?


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Jjcord2015 said:

I’m just going to replace both on that precat, rock auto has them both for about $60. Idk if they’re original or not so I’ll just do it.


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Click to expand...

From what I understand you're supposed to replace them in pairs but,unfortunately, it's both upstream or both downstream as the "pair"