running too hot?
 

in my search on this issue, everyone seems to be saying they run at 80c, no matter what the ambient temp, driving conditions, etc. -- i just went for a drive in 95f, ac on, first on the flats, gauge stayed at 90c -- then took it up hill, climbing 1000' in 7 miles, doing about 50mph average -- temp went up to 110c -- this dropped 15 degrees if i put the heater on.
new temp sending unit, and thermostat, plus i checked the temp gauge by using an ifrared thermometer at the engine, and the gauge is correct.
time for a new radiator? -- it looks fine to me, but this is an 81 240d with `180k on it.

110 deg is way too hot. I would first verify it is getting that hot. You could have a bad gauge or temp sender unit.

Then, I would start with a coolant flush. See here:
https://www.dieselgiant.com/mercedes_citrus_flush.htm

There are a lot of possibilities if it is getting too hot. I would go through each one at a time -- coolant flush, radiator, thermostat, radiator cap, fan clutch, etc.

Dkr.

90 is too hot as a steady state temp. Try replacing the thermostat.

yes, its a new temp sending unit, and i have verified its accuracy with an infrared thermometer -- it is also a new thermostat

Yes that is running warm.

Confirm that the gauge is accurate, it is full of 50/50 mix coolant. If yes replace the thermostat, if still no good it is showing the signs of a clogged radiator. Using an infrared thermometer determine the radiator inlet and outlet temp differential and shoot the radiator for cool spots indicating blockage.

Good luck!!!

One thing to keep in mind is that if your coolant is plugging the system and you just put a new radiator in, you will end up plugging it as well. I would start with draining and flushing the system to at least see how dirty the coolant is.

Dkr.

Another test you can do while the system is at operating temperature is to touch the top radiator hose and feel how hot it is and then touch the lower radiator hose. You can do this with the engine off after driving it for a while. If the top hose is burning hot and the lower hose is totally cool, your thermostat is not working properly. If the lower hose is slightly less hot, it is working properly.

Dkr.

i used the infrared thermometer to check the upper and lower hoses with the car running after returning from a test drive -- the upper was at 90c, lower was 80

the coolant has been changed regularly, and looks to be quite clear -- radiator does not look plugged -- maybe the pump?

It appears your initial post wasn’t read very carefully.

I also have an 81 240d that runs hot in the summer.
And I too have done all of the things you have done.

On a 95° day, what’s your running temperature without the AC?
Is there a different result highway or city?

The appearance of a radiator has very little bearing on whether it’s internally clogged or not.

you have the identical car
yesterday was 95, and i test drove it, stayed at 90c, ac on or off, highway/city, except when i climbed 1000', then it went to 110, no higher, ac on

since posting, i went all over the forum history, and other sites, and there is much conflicting info(as usual -- was the same experience re:ball joint replacement, which i just finished) -- found several owners who "run hot", with even the same temps i'm getting, and they took it in stride -- but then others who said their temp stays right at 80c, and anything else is unacceptable, requiring flush, etc.
never noticed this issue until this first summer here in nm mountains, so it may have existed for years, since i don't think i would have noticed running at 90c

re: the rad, i meant internally, i.e., looking down into the tubes of the core with the cap off -- i've had rads that were gunked up on other mercedes, this appears clean, as does the coolant when i change it

Try another radiator cap before loading up the parts cannon. If the cap isn't holding enough pressure, you'll build steam voids in the cooling jacket that will greatly impede the cooling capacity.

good idea, will do

Side note: what temperature is the line between 80c and 120c? I haven't been able to establish that definitely.

I would normally suspect a bad fan clutch, but I don't think the 240's have one. If it does have a fan clutch, there is your problem. If not, I agree with others on a clogged radiator.....Rich