Almost for certain it has lost some efficiency due to scaling internally and corrosion of the external fins. If it is plastic replace, if metal have the tanks removed and rodded if it isn't overly clogged. Dipping/boiling doesn't cut it IMO.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

On hot days, 28 C/ 85 F and above, I run at about 100 C at anything over 65mph. With A/C on, it can creep up to 105 C. Around town, regardless of temp, 82 C solid.

Fairly certain I'm running the original Behr/Mercedes radiator.

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1981 240D 4sp manual. Ivory White.

You've done citric acid flushes, correct?

Did you put the tstat in boiling water to check the opening temps before you installed it? I had a similar issue with my 83 sedan a few years back - new thermostat opened higher than advertised and the car ran around 95 to 105, based on ambient temp and AC usage. Replaced it with a 10 year old Behr tstat off the shelf - it opened at 82ish C and my sedan runs at that temp, as usual.

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes

Hey dude -yup, done citric flushes.
Thermostat is good too -always boil checked before install.. If it wasn’t, logically I’d be running hotter in all conditions (currently rock steady at 82 C unless under high speed load).

By the sounds of it, signs point to old radiator that can’t keep up under certain conditions.
Def visible corrosion on the exterior fins

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1981 240D 4sp manual. Ivory White.

Good thermostat and high temps on highway is an inefficient radiator symptom as long as shroud is in place.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

As much as I agree with Sugar Bear I wonder how well Shern did the coolant flush .

My '82 240D's cooling system was really bad and it took me repeated flushes to get the temps down to where I like them .

100 degrees C with the AC on isn't hot for what that's worth .

Try this : get it hot and up to speed with the AC one, once it reached 105* C, turn both heater valve knobs to full heat and make no other changes ~ if it drops the temperature, your radiator is indeed suspect , it may be bad, it may simply still have crud in it or else where in the water jacket .

I agree , the Harbor Freight infrared temp tool is really handy .

? Have you ever loosened the radiator and looked in between it and the AC condenser ? . there's always a normal accumulation of leaves, dead bugs and oily gunk in the dead center of it where the air flow is biggest .

Easy to clean .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Nate, started a new thread here: http://www.peachparts.com/shopforum/diesel-discussion/412453-pressure-testing-cooling-system.html

Re: flush, I did it as well as it could be done provided one followed the directions.
My system is quite clean. I’ve taken it apart pretty extensively and never seen any silt at all.
Anyway, I’m not convinced this is it.

Re: heater core, yes that is the technique I use when I’m not liking the temps I see climbing the grapevine.

Re: leaves and crud, i’ve taken the thing out many times to degrease the exterior and remove debris from the condenser.
You’ve seen my engine bay.

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1981 240D 4sp manual. Ivory White.

I'm not sure it's running too warm @ 105C* ~ there's a reason Mercedes put the red part of the coolant temperature gauge way up at the very end .

Thanx for the link .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better