Checking for proper working is not that simple.....
it should be overriding at speed where the incoming air is going faster than it would be pulling air in.... and then engage well at idle...where you are having your heating spikes...

edit.. if you were to replace it.... the FSM says when stored it is supposed to be in its working position... good luck confirming the parts house knew and followed that one....

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Do you think changing out the thermostat and replacing the coolant is a good idea? And yeah, I'm worried about a proper citrus flush before a long drive.