Temp gauge low
 

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.

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

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.

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.

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........

Thermostat opening too soon?
 

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...

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.

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

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

Is running cool uncool ?
 

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?

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