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

The only really dumb question is the important one you DID'NT ask.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

When my thermostat failed recently, my coolant temp stayed around 60C. With a new (used) one, it's back up to 80-85C where it should be.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Your temp gauge shoule be at or over 80 at normal operating conditions. If that's the only problem, you don't have a problem. My SD runs at 82 or so in the winter and 90+ in the summer - it's within the safe range.

I sympathise with you on your t-stat housing - I had screws break off as well. I drilled them out and used a nut and bolt all the way through to hold it on.

Don't worry about being a newbie - we all were at one time. I still am humbled by the lack of knowledge I have and the wealth that others on this forum have.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

Thanks for the fast replies and advice.
I am worried that my temp gauge is not showing an accurate temp reading, thus the panic when it went up so high after replacing the old thermostat. I should probably get the car temp tested and go from there. Do you think I should put the new thermostat back in? I always thought a bad thermostat will cause overheating. Live and learn, he?

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1982 Mercedes-Benz 300SD 210k miles

put the thermostat in hot water to make sure it is functioning right before you install. 80-90c is about where it should run. You might want to remove your radiator and give it a good cleaning but by no means at all should you take your neighbors advice.

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'85 300 Turbo Diesel- 308K

If your temp. guage is reading about 80 it doesn't have a problem.

By the way, what does the neighbor drive?

With that mileage, you are only at mid-life.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I don't run thermostats in any of my cars and I never have. I simply use a 'hollowed-out' thermostat body, which serves as a sort of restrictor, but will never fail and overheat the engine.

I find my heater performance hot enough to be uncomfortable and engine coolant temperatures hovering around 160° to 175° on a 100° day.

Our coldest winter day here is about 30°F and the average is in the mid 40° range overall.

My 440 Cubic Inch Pontiac with 9.00:1 compression runs at a consistent 155° on 100° days and doesn't get over 175° in traffic with the A/C on.

Never had any adverse wear issues and oil/crankcases are just as clean as those engines I have disassembled that ran a thermostat.

I feel your pain over the broken bolts....

Another one of my modifications on all my engines is to replace ALL the bolts/studs/washers in the Coolant and Exhaust Systems with Stainless Steel and a liberal application of Wurth High-Temp Antiseize Lubricant.

On my 300SD, I actually changed every bolt/nut/stud on the outside of the engine over to Stainless Steel, including the Motor Mounts, Vacuum Pump, Oil Pan, and Accessory Mountings. All the washers were replaced with Stainless Steel too.

I even found Stainless Steel Acorn Nuts for the Camshaft Cover...Bert