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  #1  
Old 09-26-2003, 10:31 AM
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71C Thermostat ?

Hello

I have always thought the standard thermostat for MB was rated at 87C,
but ran into a friend who told me that in warm climates, MB now recommends that "everybody" use a 71C .
can anyone confirm this ?
My car is a 126- 1987300SE , which recently began running a little hot,(no redline...)

thanks in advance
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  #2  
Old 09-26-2003, 01:04 PM
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This just sounds like the same old wives' tale. Please understand that the thermostat controls the LOWEST engine operating temperature. The fan control algorithms establish the highest operating temperature. Retain you OE thermostat. Installing a colder thermostat will NOT lower engine operating temps in hot weather, especially in stop and go driving, where the engine will heat up to 100C or more.

At 100C the themoclutch on the engine driven fan "engages" and the auxillary electric fans engage high speed at either 105 or 110.

You want at least 80C to keep moisture from condensing in the crankcase; 71C is too cold. In addition to allowing more condensate, a low temperature themostat will decrease thermal efficiency by removing too much heat from the combustion process, which means higher fuel consumption for a given operating condition.

As radiators age they build up deposits that decrease their heat transfer deficiency, so it is common for a car "to seem to run hotter" as it ages. You can minimize the rate of internal radiator deposit build-up by using MB antifreeze (Zerex G-05) or a Dexcool certified blend. Using the traditional green antifreeze will build up deposits much more rapidly because of it's high concentration of salts, especially silicates. Dexcool has a pure organic corrosion inhibitor package, which has no inorganic salts whatsoever.
MB antifreeze, which is commercially available as Zerex G-05 is a "hybrid". It has an organic inhibitor package with a very small concentration of silicate.

Duke

Last edited by Duke2.6; 09-26-2003 at 01:11 PM.
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  #3  
Old 09-26-2003, 02:08 PM
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Yup, what Duke said. Don't use a 71C thermostat. No good.
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  #4  
Old 09-26-2003, 06:37 PM
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thanks for your valuable input. - but just FYI, the thermostat I was shown is OE - It is made by Behr, which also makes the radiator I have in my car.
I have been using MB coolant quite successfully for the last few years and plan to stick with it.
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  #5  
Old 09-26-2003, 06:57 PM
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I just bought a new 87 C thermostat, my cars temp is still a little high(despite a radiator flush) so I bougt a 79 C thermostat. I am in Miami, FL where its always hot, will a 79 c thermostat make a difference in the engine temp??
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  #6  
Old 09-26-2003, 09:11 PM
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As I said before, and I will say once again, the rated opening point of the thermostat establishes the MINIMUM operating temperature and has ABSOLUTELY, POSITIVELY NOTHING TO DO WITH THE MAXMIMUM OPERATING TEMPERATURE unless it malfunctions by sticking closed or not opening all the way.

Whether you have a 71, 79, 82, 87... whatever, themostat installed, once full open, the temperature will continue to climb in hot weather stop and go traffic until airflow through the radiator is increased by the fans or faster vehicle speed.

The thermostatic clutch on the engine driven fan tightens at 100C, but idle speed doesn't create much air flow. When the electric fans engage at 105C, the airflow through the radiator is substantially increased, even if the vehicle is stopped, and should quickly drop the temp to the fan cutoff point.

If your temp gets to 110C you should check for proper operation of both the engine driven fan thermo clutch and electric fan operation. If this temp is achieved with proper fan function then radiator tube deposit buildup is possibly to the point where the radiator cannot keep up with the heat transfer requirement no matter how much air is pushed through the core.

Water flow tests mean nothing. The only way to determine if the tubes are clogged is to remove the end tanks and inspect, or just replace the radiator if all other cooling system functions check out as okay.

Duke
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  #7  
Old 09-27-2003, 02:21 AM
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All thermostats open over a range and 10-15C is typical, however, at idle the coolant flow rate is very low, so the t-stat doesn't have to be open very far for it to allow full flow to the radiator. Also, the difference in restiction between half open and full open is really not that much except at the very high flow rates that you only get at high revs.

In stop and go driving in hot weather whether the t-stat is open 1/4 or 3/4 is not going to make much difference in how much coolant flows into the radiator versus how much is recirculated back into the jacket via the bypass. It comes down to how much heat the radiator can transfer, which at a given ambient temperature is a function of air flow, and if the fans can't keep temperature in check then there's a very high likelyhood that the radiator has build up thermal resistance from deposits in the tubes.

There is a general misconception that overheating and be solved or even mitigated by installing a lower temperature thermostat. It's about as useful as putting a band-aid on a bullet wound.

The thermo-clutch responds to the temperature of the air exiting the radiator, and this usually correlates to a engine outlet temperature of about 100C in summer temperatures.

Duke
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  #8  
Old 09-27-2003, 02:32 AM
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Re: 71C Thermostat ?

Quote:
Originally posted by gidpor
...which recently began running a little hot,(no redline...)
This part concerns me. If it used to behave properly with an 87C thermostat then the problem won't be fixed but might be masked by a 71C thermostat. In 15 years you'll be looking for a 55C thermostat

The thermostat regulates coolant flow through the engine but it's not an open and shut deal (no pun intended) as someone pointed out so a thermostat of a different rating will likely change the temperature characteristics of the engine. This isn't an MB phenomenon, it's true for many engines.

Just because Behr makes the thermostat doesn't mean it belongs in a particular car. The Behr radiator cap for an 85 300SD is different from the Behr radiator cap for an 87 300SDL. But they physically interchange. If MB said that you should use a 71C thermostat then a phone call to your MB dealership should confirm it. Was your car originally destined for local (Singapore?)delivery?

Sixto
95 S420
91 300SE
87 300SDL
83 300SD
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  #9  
Old 09-27-2003, 06:12 AM
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This part concerns me. If it used to behave properly with an 87C thermostat then the problem won't be fixed but might be masked by a 71C thermostat

Sixto -this point is well taken - So is the one about the Behr thermostat being OE (in general) but perhaps not MB specific.
I am definitely not into trying to mask a problem.I would like to solve it. I was merely wondering if the current thermostat was malfunctioing . .....
I should have mentioned that the engine(103) used to operate on the 80C mark very consistently,except when under load, or stop and go traffic. This condition is now changing as I mentioned previously. This Intermittent change is leading me to think that I am getting advanced notice on an upcoming failure...The thermostat, as we know, is the cheapest link to replace and test.Also being intermittent it does not seem like a failing radiator (?)
Duke 2.6- that is when I was told about the 71C version and began this thread.
tkamiya- You are right. I know for a fact that MB has components that are Tropic rated. Tstats, Fan Clutches,AC pressure switches etc.
I am not sure about my own car.This lovely 126 lives in Tel Aviv - Israel,as I do - and was imported new most likely as Modified European/Mid East( all are) and possibly modified agin locally over time by PO's.
Now I did not succeed in taking the cap off to determine the type of thermostat that is installed in the engine. After removing the three bolts the cover did not budge. Not even a couple whacks with a rubber Mallet do it. Did I miss something ,(is the cover flush, or is there a lip ?)or do I just need to use more force.It is the Aluminum cover.
Any Ideas ?
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  #10  
Old 09-28-2003, 02:04 AM
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When my SD temp read below 80C at times and close to 100C at other times, it was an old, slow reacting thermostat.

When my SE temp read below 80C at times and close to 100C at other times, it was a stuck reservoir cap.

When my SDL temp would go to 100C and beyond all the time, it was cracks in the head

The M103 thermostat cover has a ridge that clamps down on the o-ring on the circumference of the thermostat. A rubber mallet won't do much unless you hit upwards on the snout. Or stick a long screwdriver in the snout and pry. I don't know what precautions to take if you have a plastic thermostat cover.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD
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