This might help:-
INTRODUCTION.
The maf or air mass meter or airflow meter is the biggest cause of poor performance and economy on modern diesel engines
Unfortunately, they are not a reliable component and this means that as many as 50% of diesel vehicles on our roads could have an out of spec maf sensor to the detriment of economy, performance and emissions, which is ironic because its an important component in emissions control!!.
These are the main symptoms of an out of spec or faulty maf:
1) poor performance especially below 2000rpm
2) an unexplained rise in fuel consumption.
3) flat spots, hesitation, juddering, exhaust smoke, stalling.
4) very poor throttle response - engine takes ages to accelerate to 2000rpm even in neutral due to a very weak or non existent maf signal
5) engine won't start - with no maf signal no fuel will be injected so the engine does not start.
6) engine won't rev out in the lower gears nor even in neutral
WHY DOES THE MAF HAVE SUCH AN IMPACT?
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.
The egr valve...
The ecu also uses the maf signal to control the exhaust gas recirculation (egr) system. When exhaust gas is admitted it replaces inducted air so the maf signal drops. The ecu can therefore determine how far to open the egr valve. If the maf signal is unusually high, too much exhaust gas can be admitted or the egr valve does not close quickly enough or sufficiently when the throttle is opened. This causes hesitation and flat spots as well as performance issues.
