It has been getting cold here sporadically, and I have noticed some issues with my heater not blowing warm air and had a couple of questions.

When it is cold, 30-40 for around here, the car will only warm up to about 40C. I dont think it is a thermostat problem though, because if it is in the upper 50's, it will warm up to 80C. Does this mean it is a monovalve issue, or could it be the thermostat? thanks guys

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1985 300SD
216,000 miles