1988 560SEL climate control/heating
We recently purchased a used 1988 560 SEL with 146K miles. It's a great car but the heater seems to be possessed. The problem is we lose heat when driving on freeways. Usually, stopping returns heat to the vehicle but it is unpredictable. I've tryed removing and testing the monovalve and auxiliary pump. By passed heaaaater control by unplugging monovalve. Removed and tested thermostat (compared to a new one in a pan of hot water). Currently, I'm removing the water pump to look for the problem there. Anyone had this problem? How did you resolve it?