I have a 83 300SD, and in econ mode (ec button pressed) with the temp wheel at max my heater is in full operation.Does the heat work in defrost mode (button closest to the temp wheel) it will be hot no matter what the temp wheel is set at, if that works then you could possibly have a vacuum problem with the climate control flaps and doors not directing the heat properly or the push button control could be at fault. I cannot answer if the small washer your missing in your monovalve assy can effect operation or not, only that the instructions that come with the rebuild kits say keep track/order of all parts.Did the heat work at anytime before you replaced the monovalve?