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I'd just like to give you my ten-cents worth on this one as Very Expensive Dust could cost you well over £300.
A friend has a drive-by-wire 1.8 16 valve petrol Vectra. It was well maintained and economical. It drove very well and there was no smoke in the exhaust. In spite of all this it was an MOT failure on Emissions. The problem was Oxides Of Nitrogen fractions of one percent too high! Knowing about the Lambda Sensors and thinking that these might be the problem two new ones were purchased at a cost of £90 each. Back at the MOT it was still a fail! A Drive-By-Wire expert was then hired but there were no fault codes present and the Engine Control Unit was deemed to be OK.
As with all computers Rubbish In = Rubbish Out so a new Mass Air Flow sensor was purchased (another £90!) Voila! all came good and the MOT was passed!
The Post Mortem theory is that atmospheric dust burns onto the hot wire in the MAF. This insulates the wire slightly from the airflow so more air is consumed to get the wire to the correct temperature. More air results in a lean mixture and a higher burn temperature. The higher burn temperature produces more NOX.
Allegedly it is possible to clean the hot wire by applying oven cleaner and leaving it overnight. In the morning one washes-off the oven cleaner under the tap. DON'T BLAME ME IF YOU BREAK THE WIRE as steady hands are needed.
Personally I prefer old-time diesels with a jerk-pump!
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