Solved this problem on my Mercedes 190 E simply backflush the heater matrix. That is to say run cold tap water through the heater matrix against it's normal flow. There's a thread on this on the mercedes190.co.uk forum called (190 Car Heater, And Heater Matrix Problems, Step by step possible solution.) this thread suggests using caustic soda to help loosen gunk, don't always need to though, simple back flush under tap pressure usually suffices.
Julian
December 2007
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