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there is not enough coolant in the engine to allow circulatoin through the heater core. Probably not your answr, though, since you would have other symptoms.
When the HVAC blower fan is running, air is drawn through both the a/c evaporator coil and the heater core. The conditionin of the air into the cabin depends on how much hot coolant is going though the heater core and/or how often the a/c compressor is turning on.
In your case, the a/c is turned off, right? And you are getting plenty of air flow out the vents, right?
So it is up to getting hot coolant through the core. Let us assume that there is sufficient coolant to allow circulaton throughthe core. The problem would then be that coolant is not hot enough.
Have you checked the engine cooling system thermostat? Since your year has the temp compensation board, inadequate engine temp may be masked. Has the temp gauge been running below the 9:00 AM normal position?
The labor to check the thermostat is the same as that needed to replace it. So install a new one, 92C. I use Volvo thermostats, many on the BB use another brand.
In cold enough weather, an open engine thermostat will keep the engine temp from reaching its optimum running temp. Besides reducing cabin warming, the ECU sees "Low Temp" and adds additional fuel. My 1988 244 came to me with no thermostat at all. When I installed one, the temp ran normal and the miles per gallon went up.
If this doesn't help, post back and we'll find another angle.
Good Luck,
Bob
:>)
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