So last Saturday I bravely decided to change the thermostat on our '82 300SD. There was a slight leak around the thermostat housing on a cold start. I received the thermostat and the o-ring. First thing I did was apply WD40 to the three nuts on the housing, and repeated the process (you know what is coming) I honestly looked very hard for the radiator drain plug but I still have not found it. I ended up draining the radiator by slowly letting the anti-freeze pour out of the bottom hose. Back to the housing itself. Two of the nuts were a breeze to turn, but one was badly rusted. More WD40, turn it a bit in and out. Wait. More turning and WD40. In the end the bolt broke off. Saturday afternoon and our only mode of transportation has an inoperable thermostat housing. I tried the vise grip method and the "cut a groove in the bolt and use a screw driver" method. No luck. I got a lift to the Auto place and bought an "extractor" set, and optimistically, a new set of bolts. Spent the rest of the Saturday afternoon trying to extract the broken bolt, but no luck. I learned that no amount of cussing and swearing, in any language, will help move a rusted bolt.
On Monday I borrow a friends car and drive to a local scrap yard that Willrev from this forum recommended. By a stroke of luck they have a 300D with a matching thermostat housing, dirty but in a sound condition. My first experience with "pulling" spares. After cleaning it I install the new thermostat and o-ring, fill up the anti-freeze and pat myself on the back. I go for a test drive and the heat climbs alarmingly. Drive back home and start the first of many searches on this forum for "burping", overheating, new thermostat... you get the idea. I let the car stand on an incline with the heater on and expansion tank cap off, while shaking the car up and down and singing a jolly tune, just one of the various methods I gleaned from Diesel Discussion.
On Tuesday I am sick of the car and wife takes it to work, but she has instructions to park on a steep incline. No luck, still overheating. By this time I think I have just about read every post and discussion on the subject on this forum. Wednesday arrive and after more attempts at burping the cooling system, I take out the new thermostat and install the old one. Lo and behold, the temperature is back to "normal". No need for any burping. I say "normal" because in the five years we had this car the temp reading on the gauge would be very low. There is a line about a third of the way between 40* and 80*, and normal running temp has always been on or a millimeter above that line. With the new thermostat the needle went close to the 80* mark.
This is exactly where it should be - did you think this was overheating? 
While I was looking at the many posts on this subject I thought that my aux fan might be part of the problem, so I did the test where one shorts out the a/c drier (pressure?) switch, and the aux fan did work. I got the advice to short out the switch with a 30 amp fuse to protect the rest of the circuit.
My questions
- I thought I might have an obstructed radiator, but wont this cause over heating even with the old thermostat?
- The ambient temp is around 45-55*, how long will it take before the lower radiator hose starts to heat up? I did about 10 minutes at 70 mph and the lower hose was still cool, although the engine did not over heat.
- On the thermostat housing there is a temp sensor. I read that by grounding that cable, the aux fan should come on. Mine doesn't. I am grounding it on the engine block. How can I test this cable?
Leave it be, it's a switch that prevents the heater blower from running until the coolant temperature has reached a certain level, about 52C IIRC- Last summer I started having problems with overheating while driving in town with the a/c on. Do I have a bad "ice cube" relay, or should I look somewhere else?
- Should I take my neigbours advice and take the ole girl out into the back pasture and put her out of her misery? I think he is sick of seeing me just standing there, frowning at the engine compartment. I tried to tell him that a lot of what I do is "preventative staring", but he is not buying it.
Who cares what your neighbor thinks. Learn some more about your nice car, fix it and drive it. If that doesn't pan out, sell it here 
It is probably glaringly obvious that I am a newbie, so I apologize for any miss named parts and/or terms and general ignorance.
Regards
Wessels