The air flap sits in a venturi, it responds to the velocity plus the mass of the air -- this is explained by the Bosch manual, it's the reason they used it (it's been around since before WWII on gas turbines, BTW).

It's not completely accurate, and it reponds too slowly to meet newer emissions requirements.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!