DKNEWK2, and moedip,
The radiator works by spreading the hot water out over a relatively large surface area of a thin metal barrier that has a surface area enhancement treatment, fins or a corrugated metal filler between water channels, to maximize contact with air. The air is heated by the transfer of heat in the water that in turn heats the metal of the fins or corrugated stuff.
The hottest water in the radiator is at the inlet from the engine, and the coolest water in the radiator is at the outlet. If the flow rate is very slow, the water will spend enough time in the radiator to reach a temperature approaching that of the air when it reaches the outlet. So that will give you a thermal gradient of hot to cool from one end of the radiator to the other. If there is not thermal gradient, the system is not working. You might as well replace the radiator with a short hose.
The test moedip described flows water into the radiator faster than the normal coolant flow rate. If, under those conditions you see a hot and cold spot, it will most likely be from the flow pattern (all the flow going in one area instead of over the whole radiator or being excluded from a couple of areas). The flow pattern abnormalities can be due to restrictions from junk collecting or chemical deposits, like hardening of the arteries, in the water passages in the radiator, or pinches in the coolant passages. Since the flow is not prototypical the results must be interpreted carefully. You should give a few seconds to let air vent and so on.
In any case, the moedip test quickly fills the inside of the radiator with hot water, and before it can give up a significant amount of energy to the air, you can feel the distribution of flow as areas of the radiator that are warm. Warmer areas have had more hot water flow by, and cooler areas less. I would still expect the water that entered first and ended up travelling farthest in the radiator to be cooler than the hot water entering last.
I hope that helps. Jim
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)