you need to check the valve clearances, injection timing, and go over the air way circuit, also check the fan clutch for dragging etc, if the fan is running with the engine all the time - it can cause a loss in economy.
8Km/lit is pretty thirsty, I had a LR disco with the 300Tdi engine (2.5 4 cyl OHV direct injection engine) and it used to return me 11 or 12 km/lit in city and its permanent 4WD too.