P0171 help, which O2 sensor?

[zerogspacecow]

1998 Cherokee Classic
4.0 Liter HO (157k)
Auto Tranny
4x4 with 8.25

Alrighty, so I know this is a common issue, and a lot of searching on this forum has given me a lot of good information, but I still have a couple questions I was hoping you guys could help me with.

I just bought this Cherokee a few weeks ago, and the day after I bought it I drowned it. I don't know if that's the cause or not, but recently the check engine light came on with the P0171 code ("Fuel trim to lean - Bank 1"). I'm also getting atrocious fuel economy (like 12mpg at most).

It's obvious it's running extremely rich. You can smell the gas, and if it sits in one place running for more than a few minutes, it will stain the concrete black by the exhaust. But, apparently the ECU thinks it's running lean, and so it keeps giving it more gas.

From the searching I've done, it appears that this is an issue with the O2 sensor. So, I got an ELM327 scanner to read the data. Here is what I'm getting:

From what I can tell, Bank 1 Sensor 1 seems to functioning correctly. Bank 1 Sensor 2 doesn't seem to be reporting any data. Am I interpreting that right?

Are those the only two sensors?

Which sensor is "Bank 1 Sensor 2"?

I really just need to figure which sensor I need to replace. Thanks!

 

[souske]

Sensor 1 should be upstream of the cat, this is the sensor that affects the fuel trim.

The downstream O2 sensor just measures the efficiancy of the cat

 

[Tim_MN]

P0171 (M) 1/1 Fuel System Lean. A lean air/fuel mixture has been indicated by an abnormally rich correction factor.

98 on has 2 sensors, bank 1 is Front and Bank 2 is Rear. Sensor one is first in line from the engine. Thus First Bank of sensors is front and First (only) sensor.

A lean mixture can be caused by a failed O2 sensor, a wiring problem with the O2 sensor,r or a cracked exhaust manifold. Almost all Cherokees have cracked exhaust manifolds due to a Chrysler design/manufacturing flaw.


.

 

[zerogspacecow]

Ah, so the problem is with sensor 1 then? That's the one at the exhaust manifold?

If the exhaust manifold is cracked, will it be obvious? What is the best way to find a replacement?

 

[Tim_MN]

www.ineedparts.com item#15343

.

 

[zerogspacecow]

www.ineedparts.com item#15343

.

Thanks for the link and photo!

Yep... mine looks like that.

So, I guess it's safe to say the problem is the manifold and not the sensor?

Should I get a flex pipe as well?

 

[zerogspacecow]

Alright, just wanted to give an update:

I received the new header that was linked above, but haven't had time to put it in yet.

I did however replace the upstream O2 sensor, and it seems to be running A LOT better now. The check engine light hasn't come back on (after ~60 miles), it doesn't smell like gas anymore, the MPG seems to be better based on the fuel gauge, and when I scanned it it's no longer in "open loop."

I'm still planning on replacing the header and putting in the flex pipe, but it appears that the O2 sensor was the primary cause of the problems I was experiencing.