.... with the car up to temp and running, do a check of the heater hose temp.

Feel the return hose(on pax side) and if it's hot, the monovalve is passing coolant.

Why? Either it's not getting closure power, or the rubber is torn.

Do a search. As has been said, much is written....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles