Here's an update:
I bought a new Bosch sensor from a local parts store, and put that on. Also noticed that my cat was rattling (probably from cooking itself on the rich mixture) and my muffler was rusted through.
Put in the new o2 sensor and had the cat/muffler/tailpipe replaced. 50+ miles...
I've reset the computer several times since I installed the new sensor. It runs great until about 30 miles or so, then it starts running like crap and the exhaust stinks. The problem started immediately after putting the new sensor in.
I spoke too soon. Once the ecu relearned everything (about 30 miles later), the rich condition returned. No code yet, though.
It must be the O2 sensor. It's a "Standard OExtra" unit that I got for a steal on ebay...
P0171 System too lean. Must be that crack in the manifold making the sensor read lean, causing the computer to richen the mixture... Looks like I'll get to that manifold swap this week!
I got it off ebay, but it's a legitimate aftermarket brand. Been a good month since I bought it, i just forget what brand it was.
Yes, I've disconnectec the battery several times. And a cel just showed up tonight! yay!
I replaced the upstream O2 sensor because it was having the same 2000 rpm stumble that the last O2 sensor fixed some 5 years ago. Now the jeep is running rich and idles really rough. No CEL, either. I'm aware of a small crack in the exhaust manifold (have a replacement waiting to go on) but...
Why don't the seals dry up when they're on the shelf at the parts store, then? My jeep has had a small rear main seal leak for years and it hasn't got any worse from the year that it sat.
My 98 xj sat outside on dirt in western Washington for a year during my deployment. Sta-Bil in the fuel tank and the battery was disconnected, it fired right up and I drove it home with no problems.