What is the cause of my DTC P0171 - system too lean on Bank 1?

Hi mine is a 4.0L straight 6 Grand Cherokee. I have had this code for 3 weeks now, since owning the jeep. It is LPG converted, but it runs at the same poor mpg in both petrol and LPG. I've changed the o2 sensor upstream, and checked for vacuum leaks/exhaust leaks and can find none. It runs pretty well, but can hesitate a bit when accelerating ( misfire, though no noise ), is only doing around 10mpg, and performance is terrible. There is a fair amount of black in the end of the exhaust, and actually has a ring of black around the outer edge at the end. I have started to see mentions of the MAP sensor while searching for an answer.
Any help would be greatly appreciated.
Cheers.