Should be able to change the headlight bulb with the box in place, I can....

Anyway, the box is the flow meter for the engine controls (idle speed control, air recirc, EGR) found on these engines. It measures the air flow to switch the air recirc on and off, and I guess the EGR as well.

The air recirc valve appears to increase milage a bit on the highway by reducing boost (and hence fuel use) when you don't need it. EGR is another story, most people want to disable it to prevent coked oil in the intake. However, my brother's SDL with 200,000 miles (on synthetic oil since about 80,000) didn't have any buildup when we changed the gasket on the crossover pipe a couple weeks ago, just a thin layer of soot in oil.

You should not put a K&N fliter on it because the paper one works better and is quieter.

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!