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Old 10-09-2003, 08:50 PM
dtf dtf is offline
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Location: South East CT
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Is 80C too low of a running temp for a M104?

It has been a little chilly in the mornings lately and I have had the heat on. The thermostat must be a low operating one because the temp is about 80C. Is that too low of an operating temp for a historically hot running M104 engine design? The thermostat was replaced last summer. Does the gauge read the engine temp or the coolant temp?
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (203,000)
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2006 Toyota Tundra SR5 AC 4X4 (115,000 miles)
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Old 10-09-2003, 08:55 PM
Join Date: Dec 2001
Location: Southern California, U.S.A.
Posts: 8,508
I would suspect your thermostat is not operating properly, probably stuck in the open position.

Your thermostat should start to open at 87C and fully open at 102C.

My experience and others on this site is that M104 engines tend to run at about 90 degrees.
Paul S.

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79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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Old 10-09-2003, 10:00 PM
Ali Al-Chalabi's Avatar
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Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
If its right on 80, I would consider that a hair on the cool side. I have noticed that after the thermostat gets older on my car it gradually starts to open a couple of degrees earlier. Whenever I put a new one in the engine is never below 85C
Ali Al-Chalabi

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Old 10-10-2003, 11:53 AM
G-Benz's Avatar
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
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Mine runs fairly cool (85C) in most conditions except on very hot Texas days where it might climb up to 90C or above, if sitting in traffic.

80C sounds like something is amiss...
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Old 10-10-2003, 04:39 PM
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I hate to but I've got to throw my opposing .02 cents in here. I run a 82'C thermo in my 95 E320 and it will run to as high as 90, in the middle of the summer, running the air, stuck in traffic, and around 80 in the winter. We are lucky enough to have to pay $25 each year for an emission check here in Atlanta and last year during the cooler months my car blew as clean as a new one running at 80'C. I saw no decrease in MPG, my only intention was to cool it down a bit during the hot summer here because 100-105 was not an uncommon sight on my temp. gauge prior to me installing the 82'.

And I'm sorry but I don't care what Mercedes says, 105 -120'C "with a properly working cooling system" is just too damn hot for any car. So the 82' thermo changed all of that with no ill side effects.

Now if you just want to run your car hotter than go for it, but if your ok with 80, than I can assure you the car is not running rich and it will not hurt your gas mileage.

Now would I go below 80? Probably not because I don't want to go too far down on the temp. scale to where the engine doesn't fully warm up and run clean. It's a double edged sword because too cold will cause increased wear and your gas mileage/emissions will both suffer. Too hot and your timing gets retarded, performance suffers, your cooling system is stressed out, oil will degrade faster, along with an unnecessary baking of everything in your engine compartment.

Ok, now that I've thrown my .02 cents in here let the flames begin!!!
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