my mark 3 passat cl has just bin serviced and its still eating fuel ,doing about 15-20 mile to the gallon ,has a job to get to 100 miles a hour ,seems to tickover a bit low and revs drop very slightly on tickover like its got a very very slight missfire and i meen very slight .tryed disconecting the air flow meter and it would even start so i dont think it would be that .. the car has always bin servied on the dot and has only done 86 thousand miles .... could it be the lamber senser?