Most likely poor flow through the heater core, but the cause must be determined on site.

Check the coolant level -- low coolant will airlock the heater core. Also check to make sure the aux coolant pump is working to move coolant through the heater core -- if it's burned out, flow will be slow.

A stuck monovalve will do the same thing, although they usually stick open.

A bad CC unit may do this too, getting "stuck" on full AC.

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!