Thank you everyone for the help! I greatly appreciate it! I’ll address each one of your replies individually, in the order received.

JimF: I know that the later model cars such as the ones you own run hotter, and 100 doesn’t seem out of line, but it is not normal on a diesel of this vintage. I actually did turn on the fan manually *just* to see what would happen, although I knew in advance that this was going to be futile. It was.


Rainmaker: Please. I recognize in retrospect that my grammar was incorrect, but then again I was not an English major . My writing “I filled her up with a 40/60 mix of phosphate free antifreeze + Water Wetter” meant that it was a 40% anti-freeze / 60% water mixture PLUS Water Wetter. Seeing that the cost of Water Wetter would work out to about $75 per gallon, this form of ignorance would have been not only just plain stupid, but very expensive as well. I guess that *everything* needs to be ruled out though….


Engatwork: The way that these systems are engineered is such that the thermostat does not work in the typical manner. It functions to open the passageway to the radiator as well as acts to close off the bypass circuit at the same time. Running this type system without a thermostat will not cause ALL the coolant to go through the radiator with each circulation, whereas on (most) other cars it would.


Ymsin: Oh how I wished that were the case. But much to my surprise, I discovered that on this car there is NO thermo switch sensing the coolant temperature for fan activation. Really. It is in the wiring diagram. It kind of blew my mind, but then again, that seems easy to do lately . The auxiliary fan is switched on ONLY via the temp sensor at the A/C receiver-dryer. And it does work – the fan comes on with the AC.


Intruder: Absolutely correct about the ram air effect. But, there is no blockage to my radiator. My gas engine cars behave exactly as you describe. The diesel should not be this hot under light loads & low ambient temperatures. Diesels run lean inherently, but that is another thread altogether. It does not make them run hot in the same manner as a gasoline engine.


Yal: Yes it does have an auxiliary water pump. It’s primary purpose is not for engine cooling however. It circulates water to the heater core.


Jcyuhn: I saw the 'Pep-Boys special' water pump with the stamped sheet metal vanes. I passed on that! I can only imagine the problems with cavitation that pump would cause, although American cars have used that type since the dawn of time itself. The pump I put in does have the cast impeller, but I did not actually measure the diameter. I am desperate enough that I am going to go get a dealer supplied pump & try it out. Too bad I ***** canned my old original pump with the official star on it – the core charge is $67! Maybe Mercedes prices it’s pumps so low to act as a sort of “education” for us DIY types. In other words, “Come to us for parts, they’re not *that* expensive, and if you already tried an inferior product, we’ll bend you over a barrel for not having OUR core!” (Just a conspiracy theory for those paranoid types, as well a for entertainment purposes.)

If anyone else out there wants to take a stab at this, *Please* do!

THANKS AGAIN EVERYONE!
RTH