You have a vac leak -- you should only get air out the center and side vents in "normal" mode.

Check the heater hoses for hot on the exit side -- if the hose is hot when the temp wheel is at the click stop cold side, your monovalve is leaking and giving you heat when you don't want it.

Also, check the sight glass on the receiver/dryer for bubbles -- if you see any, you are low on charge, too.

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!