took it out on my test route(1000' in 7 miles), 90f outdoors, starting with a hot engine from in-town driving(90c), ac on, climbed to the continental divide, was at 95c on the gauge, 87c measured at the top rad hose with ir gun, gauge=85 coming back down hill -- these same conditions i hit 110 before the flush, new rad, new stat
maybe the $83 rad will start leaking, but i will say that it was a perfect fit: klimoto KLI670(about 50 coolant tubes, compared to 32 on the behr rad) -- also, if this was still a cross-country vehicle, i wouldn't chance a cheap rad.
i will attribute the 15 degree drop in temp to the rad and/or the stat -- this time i cannot see the flush as having done anything, due to the clean condition before flushing.

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78 240d: Mona
81 240D: Lola
77 toyota chinook: Carlito
93 chevy k1500: Cowboy

I wonder if it was the thermostat or the radiator.

Interesting about the aperture of the stant vs the other brand...

Something I've recently noticed is that if I'm idling with the A/C on, the temperature will gradually creep over 100. I'm not sure how high it will go as I got a little tired of waiting around. Once I'm driving again it drops fairly quickly.

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1981 240D 4sp manual. Ivory White.

but your electric fan would kick in at some point around 100 -- mine came on, previous to this work, at some point above 100 on the gauge, and bought the temp down under 100 -- also, and like any opinion, there will be some disagreement, but it makes sense to me that since my stat is controlled by the temp of the coolant, not the temp of the engine block, then the coolant temp is the more critical temp -- in which case, as i noted, when the gauge was at 95, the upper hose was 87, with what i expect was a partially open, not fully open, stat -- if so, that leaves me a nice cushion until i'm in red line area, and this after climbing 1000'

in my case, i'm thinking both the stat and the rad are giving me this improvement -- i would have replaced just the stat, but when i saw the epoxy repair on the rad, i decided it had to go -- the stat switch is fairly easy, and when you have the stat housing apart, you get a look at its innards, and can see if its needing a flush

and if this hadn't helped, i would have tried an experiment: https://www.dieselgiant.com/thermostatreplacementandmodif.htm
take a spare stat and do as described, then swap it in -- if the temp drops nicely, keep it -- i can't see how it could cause harm, other than running too cool

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78 240d: Mona
81 240D: Lola
77 toyota chinook: Carlito
93 chevy k1500: Cowboy

That is a good point. Next test, I’ll wait till 110(should it arrive).

Ah the thermostat mod... the source of many great flame wars.

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1981 240D 4sp manual. Ivory White.

I too tried this 'stat mod on my first attempts to get the gauge to remain around 80* C .

I discovered that a small, like 3/16" single hole will help bleed out the air bubbles and not cause too cool running .

After doing a full and proper citric acid flush (few bother to drain the block or run the heater full blast when flushing so a LOT of heat retaining silt remains) I no longer needed to drill the 'stats in either NA or turbo engines, I use the electric auxiliary pump running on a jumper to quickly bleed out the Klima i & II systems .

The rise and rapid falling back of your temp. gauge shows the 'stat is working, it's normal to have the temps go up as you ascend hills or speed across the Desert with the AC on 'refrigerate' ~ as soon as you top the hill and head down the other side the temp should rapidly drop, not necessarily to 82* F but drop quickly .

If the tamp. rises at idle, almost certainly the fan clutch is kaput and it's a simple thing to temporarily hard wire the electric pusher fan , if this drops the temp at idle, you know what's what, if not there's either a 'stat opening problem (I too got several bad new German ones, even one from the dealer) or some silt still in the block, there are many deep corners it settle in and doesn't want to come loose easily .

If ever you have a welsh plug out of the cylinder block, probe deep inside the water jacket, you'll be amazed at what you can dig loose .

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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

IIRC, there are openings low in the engine compartment to improve airflow on certain models. They're controlled by sylphons. I don't recall which models had this feature, and I no longer have a Benz, so I can't check. If your car has them, they will be inside the front fenders, just above the rail, and will look like closed venetian blinds. I've never seen them mentioned in any discussion or FSM, and I've never seen replacement sylphons. If your car has them, it may help to wedge them open.

Nate, we're talking 240s here, no fan clutch.
You have a 240 right?

What's the issue with temp rising at idle?

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1981 240D 4sp manual. Ivory White.

i know, i was hoping i could slip that one in without anyone noticing

just came back from test#2, and i'm gonna quit while i'm ahead -- climbed from 6000 to 8400', with steep, tight 10mph turns, mid-90s temps outside, ac on, never went over 95c -- thats good enough for me

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78 240d: Mona
81 240D: Lola
77 toyota chinook: Carlito
93 chevy k1500: Cowboy

Correct and my 240d never overheats idling with the AC on, in gear, four people in the car in 113* F out side temps .

It eventually gets up to 100* C or so and the AC dash temps rise to 65+ * F but as soon as I begin moving again it cools back down and on the freeway I often get 48 ~ 52 * F out the dash vents with out side temps near 100* F .


Like a tachometer or oil pressure gauge, the entire cooling system is dynamic and constantly in flux .

Diesel engines are compression ignition and LOVE HEAT ~ that's why the intake intercooler is so popular : it cools the intake charge, not the actual engine .

Mercedes put the red zone all the way at the top end of the coolant temperature gauge for a reason....

Don't try to out think the engineers who made the same thing for decades....

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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

I do an occasional test of my electric fan by putting cardboard in front of the radiator and running up the idle a bit. My fan engages at around 110 deg.