An easy way to check if there is air in the heater core or other problems is after a high speed drive with heat on, pull over and feel all the hoses going in and out of the fire wall. If they are cold, there is air or blockage.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked