I had a similar problem with the daughters I6 WJ and I discovered after much troubleshooting the camshaft sensor drive was badly worn causing severe timing scatter, the result was lots of smoke at idle initial rough running occasional backfire through the intake and sometimes a 'fluffy' engine response. Gas milage around 15L/100km. Replaced the cam sensor drive and engine stopped smoking and again runs nice and crisp, no roughness or backfire and the gas milage improved to around 12.8L/100km.
Then on a recommendation of overseas jeep forum post I installed a new genuine mopar front O2 sensor and as expected performance did not change but gas milage improved to 11.3L/100km the best it has ever been.
The cam sensor drive was badly worn that is if you removed the sensor from the top of it, the inner shaft could easily be rocked from side to side with lots of freeplay. Now I know XJ's has had both distributers and cam sensor drives depending on years but cam timing scatter could be the cause of your problems.
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