Gilly is correct in that the thermostat is necessary to ensure the required flow, based on the heat load, goes to the radiator. An MB thermostat does more than one thing, so it has more than one set of "seats" to regulate flow. Leathermang posted a couple of schematics on this and I think I did once too. The thermostat starts with the flow through the radiator blocked and all flow is bypassed around the radiator path and back to the water pump to be recirculated. As the coolant temperature warms up the flow through the bypass line is throttled and the path to the radiator opens. This mixes cold water from the radiator with warm or hot water from the engine before it goes to the hot parts of the engine. When the load increases, eventually all the flow in the bypass line is shut off and all the flow goes through the radiator. Taking the thermostat out without blocking the bypass flow path off completely will result in insuficient flow to the radiator, as that path has similar or higher flow resistance than the bypass line. So, don't take the thermostat out unless it is not working. In that case, it may be better to remove it, depending on how the thermostat failed (like blocking all the flow to the radiator would be worse than allowing some flow). Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

It's no "theory" that the T stat is required to regulate flow through the radiator as well as the engine, it's a fact. I mean. as factual as things get. People say man never went to the moon too, but any sane person would call it a fact....hmmmmmmm, I hope I don't start a flame war. Well, someone grab an auto tech manual or something that would prove or disprove this, I'm not buying the "Delta matrix diode" or whatever. I have FIXED overheat problems buy replacing BROKEN thermostats (never able to close) before, so I know a MISSING thermostat can cause an overheat, so what does the "law of physics" say about that?
If the hot coolant is not allowed sufficient time in the radiator, not enough heat will be carried away by the passing air.

Gilly

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I am with JimSmith and Gilly on this.... ( good company I must say)

PEH, have you ever seen water pumps available for cars where they asked whether the car had air conditioning... and the airconditioned car ( more load) had fewer vanes on the water pump ? That is meant to cause slower flow through the radiator.

Hmmm, I agree with the principal of the cooling system being different in an A/C vehicle but if there are fewer vanes wouldn't that transfer at a higher volumetric rate? (since the pump is spinning at the same speed but has more fluid between vanes) Since it is easier to cool a lot of coolant a little than to cool a little a lot, they'd be aiming for faster flow under higher demand.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Jeff,,, you must be a machinist.... where the fewer cutting blades on a mill tool mean you can take a bigger cut ....
But at the same rpm I think less fluid will be thrown outwards with each turn .... if you reduce this image to one blade compared to two...or three ... etc... I think you can see that fewer vanes means less moved per turn.

Leather et al,

Saying thet slower flow thru the radiator will give more cooling is like saying a lower wind speed will give less of a chill factor.

The hotter the radiator, I.E. the faster the hot coolant is flowing thru it, the more BTU, or calories for you that are metric oriented, the radiator will dissipate. That's why a water pump is used. Thermosyphen will not move the coolant fast enough to cool engines burning more fuel.

P E H

PEH,

The point is if there is no thermostat in an MB system, too little water will make the trip through the radiator and too much water will go through the bypass port. Leading to higher operating temperatures.

It is also true that for a given set of circumstances, such as flow path resistance, flow path length, air temperature and air flow, the outlet water temperature from the radiator will vary with water flow assuming a constant inlet temperature to the radiator. In general it will vary in proportion to flow, with higher flow rates yielding higher water temperatures. Lower flow rates will yield lower temperatures as the same inlet temperature, and the same differential temperature between the air and the inlet coolant flow temperature condition would exist for both cases. So the longer the water spends in the radiator being cooled the more heat it loses and the lower its temperature gets.

Sending more hot water into the radiator will increase the efficiency of the radiator and may reject more heat to the air, which I believe is PEH's point. But it may not achieve the balance of by-pass flow at a given temperature mixing with water cooled by the radiator that the system expects at a given coolant temperature in the thermostat housing. Especially if there is no thermostat in the housing.

The MB system is designed to throttle flow to the radiator and through the bypass line at nearly all conditions except full throttle, on a hot day, with max load going up a very long hill. If this condition exists where you live all the time, well, you might be best off with a plate welded over the bypass line since you probably don't need heat often and can live with cooler engine coolant for longer during start ups. You also probably don't need to worry about -20*F water being injected into the engine when the thermostat cracks open with the engine near normal operating temperature in the dead of winter.

If you live in places with temperatures in the range of what exists in Germany, with summer trips to Italy and Greece and Spain, which is common, you should leave the thermostat in, and if there is an overheating problem, you should try to find and fix the problem. Removing the thermostat won't solve that problem.

I read here by someone recently that the thermostat seat in the thermostat housing can become corroded and leak enough to prevent shutting off the by-pass port, giving a similar result - too much flow through the by pass port and not enough flow through the radiatior leading to overheating. Changing the thermostat won't help in that case. I have noticed the seat in my old 240D is pretty chewed up and if I keep the car for much longer I will see what a new thermostat housing costs and test the idea as my 240D runs hotter than it used to run by about 10*C. Flushing, changing sensors and so on has not helped. Neither did a new, boiled and verified thermostat. So, that leaves the housing. Maybe I'll JB Weld the seat to build it back to a solid sealing area and see how that does first.

Hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

over time and miles radiators loose their effeciency. this is due to corrosion and buildup.
if you were to check the temps on the cooling fins you will find the temps in the area of the fan will be cooler.
this is due to less heat transfer to the fins.

i have seen this condition many times in older vehicles..

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79 240D my current toy
42 years a Diesel addict

240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold