quick update to my previous post. when i run the car up to temp, with the cap tight, and then shut it off, the bottom 2/3 of the radiator is cold, the top 1/3 is red hot. (radiator is new). now, if i loosen or remove the cap, the radiator temp is even from top to bottom. could a weak water pump cause this? just unable to maintain proper flow under pressure? god i hope it's not a head gasket. local mercedes mechanic quoted me about 1900 plus extras if it needs a head gasket. car isnt worth putting that kind of money in.