A very strange aspect of monovalve failure or deterioration, is that the heat might work sometimes. In my 300D, I could be driving along with heat working fine. Slight adjustment, heat gone completely, never to return until a new fresh start, when the car is cold, maybe.

After replacing the monovalve insert, that is the first thing I will always check when I have a heat problem on a 124 or 126.

Alan Hamm