Dear Denis:

My 300SD's cooling system behaves similarly. However, I believe that's perfectly normal because MB cooling system uses a bypass system. When the cooling temperature is around 80C or more (depending on your thermostat), the thermostat gradually starts blocking the bypass hole, forcing hot coolant from the engine block to flow into the upper radiator hose and move to the radiator to be air-cooled. If you do not run your car long enough (usually 15 miles+ or 20 minutes +), there isn't enough hot coolant to move to the bottom of the radiator and into the lower radiator hose (remember that the hot coolant is continuously cooled by the radiator fins when it's moving down the radiator). Therefore, the lower radiator hose will always be slightly warm while the upper radiator hose is actually hot if you don't run your car long enough.

If you don't believe my "theory", just try to run your car for at least 30 minutes + or 30 miles +, then you will see that the lower radiator hose is just AS HOT AS the upper radiator hose.

Hope this helps.

Best regards,


Eric