It is 115 degrees here in Phoenix today. My 1981 240D, with the AC on stayed at a constant 110 degrees C driving on the freeway at 65 mph.

Is this safe? Will running this hot cause any damage? When it is cooler outside, it will stay closer to 100 degrees, and on a nice day (when I do not have to use AC) it will stay around 90 degrees.

I dont know if this is normal becuase of such hot outdoor temps, or is this a serious issue that I need to start troubleshooting.

Thanks for any suggestions as always!

1.Is the gauge accurate?
(I'm not familiar with the upper limits of the 240D engine)

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'84 300SD sold
124.128

My gauge shows red at 120 degrees.

I believe it is accurate, as it shows much lower temps when cooler outside and not using AC

You should be OK at 110C, just keep an eye on it. Make sure all of your hoses (radiator and heater) are in good condition -- there's a lot of pressure in the cooling system at 110C. You don't want one to burst. Make sure the radiator is clean, all of the usual cooling system checks you probably have already done.

After driving the car with the a/c on in those temps, try idling the engine with the a/c off for a few minutes before shutting down the engine, see if you can bring the temp down to 100C, as a precaution. I know it is a temptation to turn off the engine and flee for the cool indoors (I lived in Tucson for four years).

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

*F = *C x 1.8 + 32

Water freezes at 32* and boils (at sea level) at 212* Farenhite.
Water freezes at 0* and boils at 100* Centegrade.

So 115*C is about 230*F, as long as your cooling system is pressurized so it doesn't boil out you should be fine, that's not enough heat to dammage oil or mechanical componants.

100*C is 212*F or atmuspheric boiling point at sea level.
90*C is 186*F which is what your average automotive thurmostat is set for.

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82 300SD AKA The Dieter Beater

85 GMC Jimmy AKA #$%((@

Both on WVO

It should be around 90-100c. My cars both run 80-90 in 100 degree weather with ac on. Do a cooling system flush, replace thermostat, maybe take the radiator and get it cleaned, and replace all hoses as a precaution. Im not saying its bad for your car its just not right.

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1981 300SD 512k OM603

Well, it's not "too hot" but it's also not optimum.

I happened to be diving my 240D across NM and AZ today (I'm on my way from TX to CA this weekend). Driving on the highway (trying to stay under 65 mph) I never saw my temperature exceed 85-90C. The reason; I replaced my radiator last week. The can had been exceeding 100C on 70F days in denver at highway speeds, and I was expecting to use it in hot climates. The operating temperature immediately decreased by 10-15C with the new radiator. About $300 and 45 minutes of work, a very good investment.

Check it with an infrared thermometer. It may be reading wrong.

Mine is. Mine is consistently 10 degrees C off the real reading of the actual hot coming off the block back to the radiator. It consistently shows 97-100 and even slightly above 100 a few times.... when the real temp is 85-93C by Infrared reading. It will do this in spite of a new Radiator, Coolant change, Temp sender, Overflow reservoir cap and Thermostat. It has decided to lie to me consistently and thats that. I know its wrong now. Fortunately... its fairly consistently wrong by about 10 degrees.

So check it... it could just be the gauge.

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1983 300D-Turbo - Deep Blue w Palomino MB Tex (total loss in fire 1/5/09 RIP)
1995 E320 W124 Polar White/Grey Mushroom MB Tex
2005 F150 Supercrew - Arizona Beige - Lear topper
1985 Piaggio Vespa T5 - Black and Chrome

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