My first post: this must be addressed somewhere in the old threads but I couldn't find it. My 82 300SD (617?) is running at about 78C. Now, I'm in Massachusetts so it's about 35F outside and I have the heat on. Turning the heat off only brings it up to 80C. I remember in the summer, the car usually ran at 90C with no AC on. Is it ok for the running temp to be so affected by the weather and heater usage and is 78C just too low for good economy and engine protection?

I'm mainly asking because this is my first winter with the car (but not my first Mercedes, previously 1967 200D). I bought this car knowing I was inheriting someone else's problems so I count on everything needing to be replaced. I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately (tie rods, center link, valve adjust, ALL filters, oil cooler hoses, seat switches, air cleaner brackets, steering stabilizers, auxilliary fan, etc.) Thanks!

Welcome aboard. Assuming no one's gone and swapped engines, your 82 300 SD does have the OM 617 engine. The proper thermostat for that engine is one that opens at 82 degrees, I think. I'm sure that you shouldn't be running @ 78 if the engine is fully warmed. If your temperature gauge is accurate, its time to check the thermostat. When the engine runs cool, you lose some mpg and increase engine wear. The themostat is a very easy swap out if it needs to be done.

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Tjohn

82 300 SD
77 450 SL (gone)

Get someone with a laser type temprature probe. See what the motor really is at when it reaches operating temp. Too many variables, like thermostate opening early, temp sensor going out of tolerance to a temp guage that flat out reads wrong.

Would not be the first time a temp guage was wrong.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

If you really want to know the engine temp you will need to get an infrared temp tool. The temp sending unit can read off, although it usually reads low with age. Even then it is still very possible that it reads low.
I know, I had a shop run the infrared on mine, the temp guage shows it running a bit low, about 80C. Did it a no charge. Showed the actual temp was 85C.

Mine did that a while back and it was the thermostat that was still original. It took me about 15 minutes to change it and now it runs nice and warm (not too hot ofcourse)...

the thermostat itself is like 10 dollars or 12 so it's not biggie.

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Audi TT

As I mentioned on another thread, my '82 300SD runs at a steady 60C, according to the gauge. I have a new thermostat on order, and this time I'll test it on the kitchen stove before I install it. The nights have been getting pretty cold in Southern California these days!

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1982 300SD

"I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately..."

What follows is just one opinion...

While 90C is a bit high, and 78C seems a bit low, there's no immediate danger, and your engine will keep running while you take a well-deserved break.

Watch the gage to see if it goes even lower when outside temps dip further.

Find/borrow an IR temp sensor to follow up on other posts, and to double-check your gage's calibration.

Best Regards,
Jim

Oparrott,

A large piece of corrugated card-board can serve as a very good temporary improvement for the winter months. Usually only need to cover the drivers side of the radiator, just slip it down between the A/C condensor and the radiator. Should work fine in your area until spring, but don't forget that its there!!

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150