Something obvious that might be overlooked is taking up slack in high idle adjust cable at screw fitting where it grabs linkage..... Am assuming the SD carries the same high idle adjust knob as the 240D.

Otherwise, an old trick for quicker warm-up during cold weather is carboard piece in front of the radiator.

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'80 300SD/ w116
'79 240D 4-spd
'71 750cc Guzzi

previously owned:

'83 240D 4-spd
'77 280SEL 4-spd
'74 280/8
'72 250/8
'65 220Sb 4-spd
'63 220Sb 4-spd
'63 190c 4-spd
'61 220Sb 4-spd
'60 190b 4-spd