W124 Thermostat Housing Replacement
 

Plenty has been written about replacing the W124 radiators before the plastic neck cracks and fails catastrophically.

I did that, but it wasn't enough. Also beware of the plastic thermostat housing that came on the M103 motors. Mine (1991 300E) was original, and cracked through at one of the bolt flanges, bleeding coolant everywhere.

The replacement part is now cast metal, and costs less than $20 at the dealer.

I was lucky I caught the coolant leak before it overheated. Very cheap insurance to replace the old black plastic part with the grey cast metal part next time you change your thermostat or work on your cooling system.

Plastic radiator/thermostat housing
 

Good point. The plastic radiator neck in my 86 300E broke at the upper clamp. I replaced the radiator and the original plastic thermostat housing. I was surprised the new cast thermostat housing was offered at a reasonable cost from a dealer.