Check your radiator cap to make sure it isn't maintaining the system in a vacuum. I've seen radiator hoses sucked flat. Also check for proper coolant level.

Otherwise, you have a circulation problem. The monovalve isn't working correctly, the circulation motor is bad, or you have a partial plug in the heater core, preventing circulation when the circ motor isn't running.

And, there is always the pushbutton control unit!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!