If the fan pulls air at 40 in top gear, it will continue to do so at higher speeds until it is no longer effective for aerodynamic reasons. It's generally more necessary at low than high speeds because a small tail wind at low speeds will result in near zero airflow at the radiator.

The visco clutches pull quite a bit more air than the fixed ones, because they can be "turned off" when not needed. This saves about 10 hp at highway speeds, with concommitant fuel savings.

The intake on the 4.5L is the same effective size as the throttle body since it is a venturi (note the "bell" on the end) instead of a square pipe with flat walls. You only need huge amounts of air at full throttle at high speed, at which time I would bet there is a high pressure area under the hood right at the nose of the snout.

MB does their homework properly when it comes to engine performance, especially on the older models where they stood to loose considerable sales in Germany if they didn't have adequate performance AND longevity. It's very unlikely someone will improve them much with "barnyard engineering" without sacrificing something, and that includes quietness. That throttle body is LOUD without the air filter housing.

Overheating at 90F outside temp is most likely poor coolant circulation or a dead visco clutch -- I'd vote for the visco clutch. Should be obvious when the fan is pulling, it's rather noisy on mine.

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!