Dose the SD have the protruding tubes on the fire wall that are part of the system that feeds the heater cores. If it does can you fell if all of the tubes are hot?

On W123 on the suction side between the Monovalve and the Engine is the Aux Water Pump. If the suction side of the setup is clogged up like in the Aux Water Pump it would of course keep coolant from circulating. You could also feel that return line to see if it is getting hot to determine if there is circulation.

If you are getting heat to all of the tubes on fire wall of the heater core and heat returning to the engine is it I would guess your the little heater radiator/s are plugged up with something like a mouse or rats nest in that area that is keeping air from passing through it or there is something else keeping air from passing through them.
How about if there an issue with the ducts and flaps that control where the heat would go?

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84 300D, 82 Volvo 244Gl Diesel

Those heater hoses to and from the monovalve are indeed hot. As stated, heater core has been flushed in each direction.
I think my problem is the blend door covering the heater core is stuck closed. Does anyone know the location of the heater core and/or blend door?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

" Citric acid is also far more effective at dissolving lime and calcium buildup than vinegar without attacking the metal. "

This in a nutshell .

I had to repeatedly flush and back flush the hater core in my '79 300CD before I got all the rust sediment out of it ~ I'd flush until I got clear water coming out in both directions but a week later i I did it again I'd get a burst of red water coming out upon initial flush in either direction .

Air flow is very different from heat flow and should be readily noticeable .

Sadly, I don't know the various diverter doors, maybe you could get a Mity-Vac tool and simply begin testing the chambers for ability to hold vacuum ? .

I'm keen to learn the eventual solution .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I don't know about the best, but here's how I did it on a W123 300TDT:

Removed all 3 hoses from the heater core. Flushed each hose with a garden hose, making a simple seal with my hand. Repeated several times. Reconnected the hoses. Drove car until it's at operating temp. Shut it down, let it cool and changed the coolant. The last two are because hard water was introduced into the system.

I went from no heat to face melting heat.

The three hoses you disconnected were at the auxillary water pump?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

On mine when I did it I disconnected the hose at the Monovalve (hose from the Monovalve to the fire wall) and the Hose at the fire wall that come off the Cylinder head (the one that is next to the oil filter).

The other Hose on the Monvalve goes from the Monovalve to the Aux Pump to the Engine. It is part of the heating system but not the Heater Core. That is on the suction side of the circuit (returning coolant).

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84 300D, 82 Volvo 244Gl Diesel