IF the thermostat is working properly, covering the radiator shouldn't speed warm-up times. MBZ 616 and 617 diesels use what I call a "closed loop" cooling circuit.

When cold, the thermostat closes off flow to the radiator while still allowing coolant to circulate in the engine. As the thermostat reaches temperature and opens, some of the coolant flow is permitted to pass through the radiator with the remaining flow still circulating in the engine. With the 'tstat fully open, all flow is directed through the radiator.

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