The engine management system uses the maf sensor to measure the mass of air entering the engine. At low rpm when the turbo is not working efficiently and there isn't an abundance of air available, it is important for performance, emissions and fuel consumption that the air is accurately measured. If the maf signal is below par the fuelling is reduced and performance suffers, if it is too high, fuelling is increased leading to smoke, and poor mpg, as well as lumpy, hesitant, juddery running. In extreme cases stalling or bad starting can occur.
Above about 2000rpm on most car and van engines, the turbo is within its full operating range and there is a surplus of air available so the maf signal assumes less importance because the ecu can simply use predetermined fuel maps to control the torque. i.e the assumption is made that there is more than enough air available so it need not be measured accurately.
