I agree that new parts are not always as they should be, and I would check the thermostat on your car again. The thermostat controls the flow coming from the hot engine (hot water) and directs some or all of it to the radiator. In an MB it also has a side that controls cold water flow from the radiator, and mixes the hot from the engine with cold from the radiator to maintain an even, controlled temperature from one end of the engine to the other. It also avoid thermally stressing the engine in the winter when the car warms up and the thermostat first opens.
So, if the thermostat is not working as it should, it can restrict flow from the radiator. The thermostat housing normally allows flow to the radiator, and when the engine is cold, it offers a bypass route through the thermostat to the water pump inlet, and the thermostat blocks flow from the radiator. This speeds up the warm-up process. When the engine heats up the bypass flow is restricted and the flow from the radiator is enabled. In the cooling system maximum thermal capacity operating mode, the thermostat blocks the bypass line completely and fully enables flow from the radiator, forcing all the coolant from the engine flow through the radiator before returning to the water pump inlet.
If both moving plates in the thermostat are not free to move through their full strokes, it may restrict the return flow from the radiator when, based on coolant temperature it should be enabling greater flow of cooled water from the radiator. Or, it may always allow bypass flow. In either case the hot water going into the radiator at the top will be nice and cool, or at ambient anyway, at the bottom of the radiator as it slowly makes its way to the outlet.
There should be a thermal gradient from top to bottom (inlet to outlet) anyway (or you are not removing any heat), so a cooler bottom (outlet) of the radiator is perfectly ok and by itself not a sign of something being amiss.
Good luck with the issue. Many have struggled and in other threads on the subject, changing viscous fan clutches and/or thermostats seem to help. Jim
edited for clarity
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
Last edited by JimSmith; 06-25-2002 at 01:23 PM.