Strange o2 issue
 

Replaced my upstream o2 sensor for black smoke and hesitation. After replacing it the engine would not rev past 3000. After checking everything I just unplugged the o2, and it's never run better. I know I shouldn't run it like this, but I have 2 extra xj's that run. Any thoughts as to why it runs better without the upstream o2?

It's a 2000 limited 2wd auto.

I seem to remember something about a default mode that kicks in when the O2 sensor isn't there at all.

Since it's better without it, that means you have another problem that needs to be solved. Basic 'poor running Cherokee' troubleshooting is in order! That means crank sensor, cam sensor, vacuum leaks, and the like.

Is it throwing any codes?

Getting codes checked tomorrow and I'll start there. This is my favorite of the 3 I have, and is going to get some upgrades.

P0108, MAP
P0132, O2 High voltage

Drove 185 stop and go miles today without the o2 sensor and the MAP issue, and it was perfect in every way. Going to just let it ride and see what happens.

Is your map sensor still connected? There is a little rubber elbow that can deteriorate over time.

It is, but it has some surface cracks on it. Ordered new sensor and lines.

I haven't repairs it yet due to the fact it runs so damn good. All fluids look great, no visual signs of excelerated wear.