Monitor your engine temperature gauge in the morning for telltale signs.
Just how long is it taking for the engine to get up to operating temperature. If it is taking more than 5 minutes the problem could be a weak or failing thermostat. Easy DIY to replace yourself.
Problem & Solution
Common among these chassis:
Classic Sedans
Ponton
W110 W111 W112 Sedan
W111 W112 Coupe
W113
W108 W109
W114
W115
R107
W116
W123
W124
W126
R129
W140
W201
W202
W208
W210
W203
W220
W209
W211