I have found leaving the 400 watt block heater in my 300D plugged in overnight does make the engine start much easier in 20 F temps but it does not give instant cabin heat- 400 watt is not high enough for that. Plugging it in for 1 hour gives the same easy start so instead of wasting electricity I power it for just an hour before I need to start the engine..

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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