It's the anti-backrunning valve. If you stall the diesel at just the right time, it can start up running in reverse rotation, with the air coming in the exhaust and going out the air cleaner. Doesn't run very well, but will give you quite a shock when you take your foot off the brake and step on the gas.....

Naturally, the oil pump is working backwards, too, so MB puts that flap in the intake. It closes tight when the air flow is reversed, stopping the engine before the bearings go.

Not present on the later models, I think the IP cuts the fuel if the engine runs backwards, but I'm not sure.

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!