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Old 06-26-2003, 06:16 PM
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Question How temperature affects a Diesel


I've noticed that my 190D 2.2 seems to run quieter and smoother when it is 60 - 75 deg F outside than when it is 85 - 95 deg F. At the higher temps, it seems to nail more at higher RPM and load.

Anybody else noticed this? I was just wondering what would cause this phenomenom.


What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.
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Old 06-26-2003, 07:07 PM
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Me too

I've just assumed that it had to do with the increased density and decreased temperature of the charge air coming out of the turbocharger when the ambient temp is lower.
Never a dull moment at Berry Hill Farm.
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Old 06-26-2003, 08:43 PM
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R Leo,

Turbocharger, LOL. My "turbocharger" is the button to turn off the a/c

But I think your are right about the cooler air being more dense and therefore aiding combustion. The car temp varies a little (runs 85 deg C at 70 and 90ish deg C at 90 deg F outside temp), but I don't think its enough to cause any difference in injection pump function.

What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.
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Old 06-27-2003, 12:11 AM
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I believe the diff in the engine temp with changing outside air temp has more to do with your thermostat opening and closing (I have 80-90C stat while the OEM stat is 85-95). if you run your climate control, at 60F it probably kicks in heater occasionally. Heater acts like a second radiator, so coolant temp drops and the stat closes, so the coolant temp continues to oscillate between 85C and 90C (half opened).

As to running smoother the only real diff I noticed when the temp outside is below 55F, but that may have to do with the antijelling compounds added to diesel in winter. Good point about "turbo button in 2.2l
1985 190D 2.2l Sold-to Brother-in-law
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Old 06-27-2003, 12:39 AM
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Doesn't it make sense that the hotter the temperature, the thinner the oil and the more likely there is to be metal hitting against metal?

Probably not a good thing...
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Old 06-27-2003, 02:56 AM
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oil thickness and temp will be about the same after the engine is warm at any outside temps (except for the extreems). This is obviously not true until the engine has been completely warmed!

Air density is a factor when you drive easily W/O turbo, as the turbo will heat the air and compress it (increasing density anyway) and since these cars don't have intercoolers (which would cool compressed air with outside air), outside air temp means little for normal diesel driving (floored 90% of the time); except when idling of driving slowly enough that the turbo is barely functioning.

Have you considered that your torque converter or ATF viscosity (ATF is much more sensitive to temp changes and heats slower that engine oil) may cause the engine roughness? Does it run smoother only either in gear or out of gear, or in both?

Also, if you have auto-climate control and it is set at the same pt. (normal mode and cool) because the a/c compressor would not run as much in the cooler climate as the hotter it would run smoother and quieter in the cooler temp when the compressor wouldn't run as much. You'd probably notice if it's the compressor running that's causing the roughness though.

Just a thought...


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Old 06-27-2003, 06:39 AM
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Yeah, this winter when they had winter fuel, it ran a little different but I'm talking about variations from Tuesday to Yesterday: same tank of fuel.

jassz, I'd hope that the relatively small temp swing wouldn't change the oil viscosity enough to not lubricate properly. Maybe so, but I'm running Mobil 1 Delvac.

I was wondering if the Diesel viscosity is changing. The OM601 has a fuel heater that should run only until a certain temperature. Maybe mine is stuck in the always heat mode.

ATLD, Sorry, got a manual tranny (thank God with only 70 HP on tap). Also, my car has been converted to R134 so when the A/C kicks in you can surely feel it. And that certainly affects running smoothness, but the effect is present even with the a/c off.

I'm really talking about running smoothness when the outside temp is cooler but not cold. Is it possible that there is a viscosity change on the fuel so the IP doesn't have enough resistance generate enough injector pressure to fully atomize the fuel?

Or maybe the hotter air requires a different injection pump timing value and the OM601 doesn't have a computer to adjust it automatically?


What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.
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