The heater on my 82 300SD has the same symptoms as you describe. I changed the mono valve but that didn't solve the problem. I just keep the fan speed on low and the interior heats up fine, it just takes longer.
If you get yours fixed, let me know what the solution is.
J. William Tarbrake
NY State Dealer 7014303DLU
Registered repair shop R7014303
82 300SD modified for waste vegatable oil
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