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. 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?
- 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.

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

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1982 Mercedes-Benz 300SD 210k miles