I tried to locate the original comment from M.B.DOC(old post), but was unable to do so.
If I remember correctly, some older 103 motors have a tendency to have trapped air which can cause overheating.
Elevate the front of the vehicle, remove cooling system pressure cap(radiator cap), start engine and let it warm up awhile. Watch the overflow tanking for bubbling/gurgitation.
For a diyer, a good set up ramps might help with the elevation of the front of the vehicle.
1991 300-SEL - Model 126
M103 - SOHC
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