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Old 05-05-2004, 03:54 PM
The Warden's Avatar
Certified diesel nut
Join Date: Mar 2002
Location: Pacifica (SF Bay Area), CA
Posts: 2,946
Question Diesels want to run warm, right?


I was talking to a friend of mine who also has a 123, who had paid a visit to an old mechanic who had supposedly been trained at Stuttgart. The mechaic told my friend something that really surprised me.

The mechanic was advising that my friend make some sort of modification to the thermostat to make the car run cooler, and that doing this would make the car run more efficiently, have more power, and get better fuel mileage.

Now, I've been under the impression that diesels prefer to run hotter, and are actually at their most efficient at a temperature right below the point where the metal can't tolerate the heat anymore. Obviously, we don't want to run the engines at this high of a temperature (whatever it may be) because the engine won't last as long, BUT, we don't want the engine to run too cool either. This is why thermostats are installed instead of leaving the space open for the coolant to flow freely.

Do I have this wrong, or does the mechanic not know what he's talking about (or is going off an old wives' tale)?

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Old 05-05-2004, 04:19 PM
dieseldiehard's Avatar
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,092
Use the right thermostat and be done with it. Diesels run cooler than gassers in the first place, and they need to be at the right temp to provide good efficiency, that is why they lack power when first started, the pre-combustion chambers have to get to the right temperature.
'00 E320 (wifes car), '95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!
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Old 05-05-2004, 04:53 PM
Old Deis
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Diesels can run fairly cool, except when under a load. Then they put out much more heat than a gasser. Diesel fuel has a lot more energy potential than gasolene. That is why we have those oil coolers that have thermostats to open only when things get hotter, as these engines are capable of overheating quickly if say, you climb a long steep hill.
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Old 05-05-2004, 05:05 PM
Registered User
Join Date: Jun 2000
Location: Saugus, CA USA
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If you get a gasser too hot it will start running bad. If you get a diesel too hot it will start running real good. The combustion process will keep improving way past the melting point of aluminum
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Old 05-05-2004, 06:13 PM
Registered User
Join Date: Nov 2000
Location: Mustang, OK
Posts: 509
Just my observations with my 190d 2.2:

My car seems to run best (most power, smoothest running) when at 85 - 90 deg C. When it gets hotter, you can hear more combustion noise. I think this is maybe due to the timing requirements changing as a function of engine temp. I figure that if I had a knob on the dash to adjust the timing on the fly, I could make it run "best" at any given temperature.

What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.
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Old 05-05-2004, 11:43 PM
compress ignite's Avatar
Drone aspiring to Serfdom
Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
Posts: 5,554
Post regulated engine temp.

There is a "sweet" spot for each and every diesel (as well as
gasoline) engine [depending on Design, Use and MAINTENENCE.]
with reference to operating temperature.

Barring direct communication from: The Almighty , or Rudolph
Himself ; I would default to the Engineers @ M.B.

Throwing out any "manipulations" for our silly non-diesel
oriented emissions, the Engineers have designed the
optimum temperature range.
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Old 05-06-2004, 07:47 AM
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Join Date: Nov 2001
Location: North Central Kentucky
Posts: 1,065
Ok, assuming the engine is at the proper temperature, what about the fuel? Does it have a best temperature? Best atomization, easiest combustion? Should we all be running our fuel thru a heat exchanger connected to the engine coolant ...or the air conditioning? Does any of that push around the combustion temp so much that it ruins the emissions? Or helps? This, of course is for diesel, not vegie oil which is a whole different matter.
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Old 05-06-2004, 12:21 PM
dieseldiehard's Avatar
Join Date: Oct 2002
Location: Bay Area No Calif.
Posts: 4,092
The OM603 series diesels (W124, '86, '87) use a fuel heater, it uses hot coolant to heat the fuel. I believe that this could be related to increasing engine efficiency.
There is also a difference in the bore and stroke made when they added the 6th cylinder but kept the displacement at 3l, the engine has less of a square ratio of diameter to throw.
My OM603 engine also warms up a LOT faster than any OM617 I have ever driven, I haven't figured out how this was accomplished, must be thermostat related.
The IP might also be designed to be more efficient and vehicle weight and aerodynamics may also play a part in this but the fact remains fuel economy is better in my '87 than any of the W123 turbo dielsels I own.

I think that higher fuel temp might just be part of the improvement, if so that makes me wonder if modifying that aspect on the earlier 5-cyliner engines would be effective?
'00 E320 (wifes car), '95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!
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