Touch the hoses on the monovalves, ie before from engine and after to the heating radiator , if that one before is warm (be carfull temperature can reach 85°c) and the other not, there is the problem.
if there is an auxillairy pump you should ear the sound and feel pressure schocks on the hose.

Good luck

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Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class, 220CDI, 2009, W204

Sold: E-class 260E, 1988, W124, Wonderfull car sold after 489 000 kilometres of reliable services (engine M103, clutch, brake system and 5 speed manual gear box all original).

Sold: E-class, 320CDI, W210, 2000, 225 000km