Cazzzidy,

An earlier post on the subject, one of my favorites, noted the thermostat housing can be the problem. The housing can suffer from crevice corrosion under the thermostat housing support flange, leading to a perpetually available bypass port. This, when it gets big enough, can short circuit the usual flow of a partially opened thermostat, leading to an apparent loss of radiator efficiency since the flow path through the radiator is typically not a loss-less path. In the end, you do not get the full capacity of the radiator as the radiator is not taking the full flow of the system.

Another item could be your fan. The engine driven fan on a 300D, unlike a 240D seems to be controlled by a device that engages a clutch when the coolant reaches a certain temperature. If this item fails, you typically have problems at low speeds in warm weather, which are less severe at higher speeds or in cold weather.

In any case, I would be reluctant to let this search and destroy mission continue. Check the clutch thermal switch, and then the clutch itself. Then, if that is ok, check the thermostat housing, and if the support counterbore is pitted, try filling it with a material like JB Weld. Fill the pits and then smooth the surface so it is reasonably flat. Let us know if something works or doesn't. Good luck, Jim

__________________
Own:
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

Owned:
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)