On my '87 300D Turbo (W124, OM603) there is a heat-operated vent on the driver's side of the engine compartment. It opens when it gets hot, helps to cool the engine compartment. Perhaps later models had it on both sides. I know it works because on hot days, in slow traffic (stop-and-go) the outside temp gauge goes way high when the vent opens (one of the reasons Mercedes moved the sensor in later models to the front of the bumper).

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970