So I have the P0171 System to Lean MIL. I checked my 02 sensors replaced the downstream.I have a small Crack in my headers.I get about 6 miles per gallon and pour black smoke on the high revs.Can't figure out whats the problem.It runs fine until about 45 then it starts to knock and surge.but 0-35 mph it feels fine.Any Advice??

 

Small crack in the exhaust is letting air into the system that is tricking the O2 into thinking the engin is running lean. The computer sees this and reacts by dumping fuel to compensate. I would fix the exhaust and replace both Up and Down stream 02 sensors to start.

 

Its a very small crack at the collector like most xj's have stock.How sure are you on you opinion?

 

BTW I had the crack well before The MIL came on.

 

Im 100% sure on my opinion, without seeing the vehicle and doing diag over the interweb from what you have typed, Im still about an 80%. Is the exhaust only cracked in that spot or is it cracked some place else you can't see? The computer reads the up and down stream o2 sensor outputs and compares them, if the computer sees that the engin is running lean it is going to add fuel, causing black smoke and poor fuel mileage

 

100% it is right by the collector.