This is what the factory service manual says for YOUR car/engine:

"When driving under full load, on mountain roads and bumper to bumper, or following a fast ride on an express highway with subsequent traffic congestion, or when driving in areas with high outside temperatures, the coolant temperature indicator may rise up to the red mark if an anti-freeze of at least -30 deg C is filled in, without any ejection of coolant or faulty running of engine."

In other words, no worry required if you regularly see temperatures up to 100 deg C, under the driving conditions you describe.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

The thermostat has two control surfaces. It is simultaneously controlling flow to the radiator and to the engine block. If my description was poor (or backwards), I am sorry. I was just trying to paraphrase the FSM. Here is the official version... (see attachment)

Attached Thumbnails

 

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

And again, adding a 75 degree thermostat, is a band aid....you are just trying to mask the main issue.

Very true. When the thermostat is already wide open, it doesn't matter what its temperate rating is. If you want real long-term protection from corrosion and overheating, switch the system to NPG like I did with my signature car. You can also run at zero pressure, removing strain on the hoses, radiator, and heater core.

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'83 300D, 126K miles.

This diagram is much clearer.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The problem is, the FSM is actually confusing. The line that's identified as the "bypass line" is actually the main feed to the water pump. I suspect the word bypass was applied by a translator, and it's stood the test of time, however inappropriate. The term "bypass" only applies to the opening behind the thermostat.

The thermostat isn't directly controlling flow TO the radiator. It controls flow FROM the radiator to the water pump. It may seem like a semantic exercise, but it's critical to understanding the design. While it's true enough that controlling the outlet also controls flow at the inlet, the objective is supply water at a controlled temperature to the block, as opposed to dumping hot water to the radiator. In the former case, the temperature of the block will be fairly constant. In the latter case, The temperature of the radiator will be stable, but the water in the jacket could be too cold or too hot.

With the Mercedes constant temperature design, the temperature should be rock steady at whatever the design point is, but that will be a factor of the radiator's heat transfer ability and the airflow over the core. If a car isn't doing this, it's either due to a compromised radiator, fan or sensor, assuming a designer didn't become confused as we are here, and spec'd something incorrectly. And as I've said before, it may be that they compromised the design of the fan systems deliberately to goose performance.

OK, so your description is sinking in of the constant temp design.

And this is what I've found on my 1979, which has a brand new radiator that I installed as well as a new water pump and thermostat and temp sensor.

I think that most of the anecdotes about these cars fluctuating so much, especially at speed when airflow is best theoretically, are just suffering from aged components.

I took my '79 on a trip during a warm springtime 90°F spell with four people loading the car down, climbing hills in 3rd gear at high RPM, and it stayed right at 90°C on the gauge.

My '85 isn't that constant, though still doesn't jump around. I have a new radiator going in, and will do the water pump at that time. It already has a new thermostat, and the gauge is tested good. We'll see how, or if, it changes after that.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

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