Runs rich and cel
 

Hello,
I need help on my 2000 Jeep Cherokee 4.0. block was cracked when I bought it so I put a long block in it and new plugs crank and cam sensors. It's been running great for 6 months then then cel came on code for lean upstream O2 sensor and rough idle and chugging on accel when cruising and my fuel economy went from 18 to 10 mpg in 2 days. I replaced the sensor and no change. this is my fist Jeep not sure if there is a common failure with these symtoms.
Please help ...Thanks!

Check ur exhaust manifold for cracks, and also the o2 fuse located in the engine bay.

I agree with purplexj that the cracked exhaust manifold is a primary suspect for this code.

It's not the sensor itself as you already found out; the sensor is trying to adjust itself to the lean mixture, but can't, so the light and code are set.

thanks guys but no exhaust leaks, checked all fuses they are good. Also checked the MAP sensor with my vacuum gauge and DVOM and that checks good.
Any other ideas?

The code says lean condition but mpgs went down? That sounds odd to me, but who knows.
*Did you replace the O2 sensor with the original NTK? I know some have problems with Bosch or other available brands.
*If it's a lean condition, I might suspect an injector, or maybe low fuel pressure.
*Also, I've read on here that a bad temp sensor can cause a too rich condition if the sensor sends a 'cold engine' signal, but that's the opposite of what you're saying. :icon_confused: Can you say the exact code it showed? POxxx or whatever? Sometimes the code can point to parts related to the O2 sensor, but maybe not the sensor itself. I'm interested to find out what the solution ends up being.

Sound advice.. x2 or 3 on replacing all o2 sensors with OEM, the temp sensor is a good call.. Have you had your XJ on a code reader? If so how good of one? Did they check over various computer/sensor function? Without the boys down at the shop looking over my TPS, IAC, o2's etc. on their computer, I might still be running a bad sensor or two! My .02
Good luck.

The code is po171 and unfortunately it was just a generic scan tool. as for the injectors is there anyway to test them with out just reaplacing the six pack? That can be expensive. And I have a temp gauge and it seems the sensor is working correctly. Sorry not sure what brand O2 sensor is I can check.

Flow testing is pricey... Sea foam is cheap?

still have the catalist converter? if so could be clogged or if its missing that be why its throwing codes my 2000 has not cat and cel stays on with the o2 sensor code it runs great tho and gets 16 mpg there about.

OK so I tried the sea foam, cleared the code and it seemed to work great MPG went up and runs great for about 150 miles
Then cel back on yesterday with same code 171 no perfromace complaints until today rough idle and at 65 mph feels like it is slaming in a gear but it wasn't and that is the only way I can think to discribe it. It was doing this before the sea foam trick too just could not think how to discribe it.
And yes it has a cat but I would think if that was plugged it would throw a down stream O2 sensor code or both...