My 240 D, which has been running great on well filtered WVO for about 80,000 miles, still runs fine, but for some odd reason, the temp gauge has a hard time getting up to 80C. I first noticed this on an 85 deg F day. It has forever run at 80 C or a bit above on such a day. On a cooler day about 70F, it was running at more like 60 or 70 C. Never had this before. Always runs at or a bit above 80C. Any ideas? Could the gauge or sending sensor be failing?

Time to replace your thermostat.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

Could be the gauge but I'd start with the thermostat as suggested. Use a good one, it is critical.

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

My 1984 300D was running cool ~60 C when should be 80 C. The T-stat seemed to work about right in a pot of hot water, but when checked more carefully against others and w/ IR gun and thermometer, it opened too soon and responded too sluggishly. A new T-stat fixed it. Perhaps the thermal wax inside gets poorer with age.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

This was on a gasser. I had an issue where the temp sensor got covered with some sort of white build up and it effected the heat transfer to the temp sensor.

If the connector is corroded and the terminal is corroded that can also give a false reading.

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84 300D, 82 Volvo 244Gl Diesel

I had the same issue on my 78, 240D. First thing I did was replace the thermostat and temp sensor. Cured my issue. After poking around the engine I found other concerns on the cooling system so off to the races.

I replaced a leaking radiator with a known good one and also the water pump which had a seal problem. New radiator cap to boot.

Also replaced the upper and lower radiator hoses due to surface cracking on rubber.

Also had to troubleshoot the electric cooling fan to get it working.

This was after I serviced the AC unit back to health.

When I am sitting in summer traffic and it’s 110 degrees I don’t have to worry about any nasty surprises from the cooling system.

I am not against just fixing what is only broke but I have lived through several roadside debacles over the years trying to limp an old car home with an unmaintained cooling system and I finally got tired of sunburns, burnt fingers and a steam bath so every project I do now gets the attention it deserves.

I will get off my cooling system soapbox now, LOL.

Thermostat........

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

If the thermostat is opening too soon, wouldn't it eventually reach the normal operating temperature of about 80C?

^^^That depends on ambient temp, cooling system condition and size, and load on the engine. Short answer maybe...

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

Thermostats do not just "open." They regulate within a temperature range and do not fully open until that range is exceeded, which normally requires pretty adverse conditions.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

An old timer told me that if you replace more than one thing at a time you don't know what fixed the issue.

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84 300D, 82 Volvo 244Gl Diesel

Another old timer story by another old timer. Sorry, had to make a little fun of myself, LOL

I guess i will take some measurements with the IR thermometer to see that the gauge corresponds, then if that checks out, change the thermostat. It's only 41 years old, (200 in cat years).

If i put that off a while, is there any damage from running a bit cool?

Thanks for all the cool advice!

Replacing the thermostat is the cheapest and most obvious solve.
41 years old... do you really need to futz around with an IR thermometer?

Re: damage, one of the others will chime in, but 60 is pretty far off 80. You're certainly getting less efficient combustion. Your valve lash is also set for a hot running engine.
Why not get it over and done with?

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1981 240D 4sp manual. Ivory White.

As others have said - thermostat first. I've had issues with other brands lately that were resolved by a MB/Mahle/Behr.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".