ezrider and stephenson,
If the operating temperature suddenly changed, it is unlikely the radiator is the source of the problem unless there is something obvious, like a plastic bag covering it, or it is clogged.
On my 350SD I had the fans all come on driving over the mountains on I-90 in Massachusetts a few years back in the middle of a blizzard. The car actually started to get hotter than the normal 80 or 85 degrees C, so I stopped to check things out. The entire radiator and the electric fans were packed in snow. Once this happened the radiator became less effective and the car started getting hotter, approaching the red area on the gage.
I cleaned out the snow and the temperature came down, but within about 15 minutes it started climbing again. Clogged with snow all over, so I cleaned it out again. Did it twice more before I got home. The car used more of the temperature gage range thereafter, and I presumed it was the operation of the fans that was the problem until I ran the car idling (used to idle at any ambient temp and stay at 80 degrees C) and watched the fans work as the car heated up to 105 or so C. In the end I traded the car in and never solved the problem, but now have a similar situation on my 1982 240D, which I will address this summer. I am convinced it is the thermostat, based on threads on this and the Diesel Discussion boards on the subject.
For the radiator to suddenly plug up, a hose or some other part has to start to disintegrate and spew large enough particles in sufficient quantity to make the radiator stop flowing the nominal designed fluid by a significant margin. A gradual build up of chemical crud on the inside is not going to suddenly either reduce flow or change the heat transfer characteristics of the radiator enough to make such a change.
I think the number of reported radiator change outs with no effect on the operating temperatures validates this, and takes the likelihood of an internally disintegrating part out of the picture as well. A new radiator would not immediately plug up with particulate, unless the hose or whatever was also showing signs of failure, like bulging or leaking.
The same is true of the number of water pump changeouts to address the cooling issue, with no positive or net effect.
The least expensive, and most critical element of the temperature control that is active at engine temperatures of 80 degrees Celsius to 85 degrees Celsius is the thermostat. The fans don't come into play until the temperature gage reads over 100 degrees Celsius. So they cannot be the reason the temperatures that used to stay well under 100 degrees Celsius now creep up to those temperatures.
In normal operation the thermostat is never "still" as it adjusts the mix of hot bypass flow and cooled flow from the radiator to suit the load and ambient temperature and air flow conditions. Consequently this device is subject to wear. It is also substantially more complex than most thermostats as it has two moving flow control devices that sit in hot coolant from the engine on one side and cool fluid from the radiator on the other. It has been reported that one individual on the Diesel Discussion board got three or four non-functioning thermostats from the dealer in his area, and it was not until he got one that worked that his engine operating temps went back the 80 degree Celsius mark and stayed there. He even pulled the head and changed the head gasket looking for some reason for the problem. He has some head work that was due at the time anyway so he did not take the head off for this problem alone, I believe.
Since the thermostat has a number of functions, it is hard to check by boiling it and visually determining what is up. It should be feasible to boil the new and old thermostats, recording the temperatures each unit responds with motion of the two flow control elements to, and then compare the responses. Responses being rate of movement and range at the temperatures. By the time the thermostat is at 100 degrees Celsius, it should be in its full cooling capacity position, as the fans start coming on after that to increase capacity.
I hope this 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)