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Old 01-03-2002, 10:30 PM
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Location: Concord, MA
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final word on coolant temp.

based on recent posts, i changed my thermostat because coolant temp. never exceeded 75C. With new thermostat (german, Febi), coolant temps range from 90 to 95, (with outdoor temps in the 20s-30sF). Based on recent posts, I understood that coolant temp should be right around 80C.

I have recent new radiator, always ran MB coolant, new mono valve core, ACC/Heat works great....no problems whatsoever with cooling system.

So I figured maybe the new thermostat is bad. I bought another one, and also replaced the temp sensor for dash guage. Temps still from 90 to 95.

Now I'm thinking that this is normal, and I was inclined to disagree with those who say dash temp. guage should read 80C. So I looked in Haynes and Owners Manual. Here is info:

Haynes- (pg. 29) "Thermostat starts to open at 80C, fully open at 94C"

Owners Manual- doesn't give normal temps, but says that normal coolant mixuture (50/50) has a boiling point of about 124C, (this corresponds to red line on dash temp guage) and that "during sever operating conditions and stop and go city traffic, the coolant temp. must not rise above the red marking".

Based on my experience and on above info. here's the scoop:

the correct thermostat (rated 80C) starts to open at 80C, but does not fully open until 94C. Thus, normal temps for normal driving conditions are in 90 to 95 deg. C, maybe a little higher in warm weather. On a hot summer day, with AC on, with full vacation cargo, i would expect temps to go a little above 100.

For all those whose dash temp reads consistantly around 80C, your thermostat is old...throw it away and put in a new one so you will experience normal temps in the 90-95C range. Also, you will experience proper cabin heat when ACC setting is on max....nice and toasty!!!

As thermostat ages, spring must weaken, and coolant temps will decline (unless it malfunctions and gets stuck...resulting in very high temps!!!). When dash temp guage indicates temps less than 80C, its time to replace thermostat.

Mark M.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Last edited by MarkM; 01-03-2002 at 11:03 PM.
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Old 01-04-2002, 02:15 PM
Morphous
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The proper thermostat for 300D is 87C. My car runs right at 87-90 C constant. Their are vacuum components that open at or above 80C. It sound like your car is running normal to me.

Morph
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Old 01-04-2002, 05:51 PM
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Join Date: Sep 2001
Location: Concord, MA
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thermostat

For the 123 body 300D's (up to 1985), proper thermostat has temp. of 80 deg. C stamped on the housing. According to info in Haynes, which comes directly from MBZ specs. thermostat begins to open at 80C, and is fully open at 94C.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)
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