If you have a well-running W210 1998 or 1999 E300D, what do you observe as to the stable engine temp? Mine seems to stay pegged at 78C, which seems a tad on the cool side....but Quahog tells me is 99 runs a little cool as well.

I think spec is 80, so perhaps 78 is close enough.

What does yours do?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Mine hangs right around the 80C mark all the time once it's up to temperature.

If I go say 20 miles on the interstate and then exit, the temperature might climb up about 10 degrees or so (halfway between the 80 and the hash mark, so I would call that 90) while driving in stop and go traffic. Once I get moving again the temperature goes back down to hover around the 80 mark.

If you really want to know the temperature with more precision, put the climate control in diagnostic mode (hold down REST for 5 seconds) and display parameter 06 (by pushing the left AUTO until you see 06 on the left). The argument is the coolant temperature in degrees F.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Mine runs right at 78 as well, just like yours. On highway runs, it usually drops about 5 deg as well; the coolant was changed about 3 months ago, and I am running Zerex G-05

It took a bit of getting used to this temp, as I was used to the engine running at a bit over 80 deg on the 190DT. However, in 2 years of ownership this E300 has consistenly maintained this slightly lower operating temperature.

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'98 E300 DT
'87 190D 2.5 turbo RIP
'92 S500 Sold
'03 3.2TL-S A-Spec

Under 80C. Whether it is "78C" or not, I would think it would need to be calibrated digitally to know? Over 80C when AC is operating in City stop n go driving on those +95F days.

Using the Scan Gauge, it is consistently between 168-172f, this is for highway driving and "light" stop & go. IN the summer with the a/c & more stop than go, it will be higher.

Our '98 ran at 78C as well. I fitted the hotter thermostat, 85C, IIRC. The needle now suggests it's running around 81C on OE coolant.

My 1999 car always ran at an indicated 80C.

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1998 W210 diesel (wiped out by a texter)
Baum spring compressor "for rent"

The 99 runs consistently at 85C and 95C (with AC) in city stop-n-go even with the head gasket leak.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Between ~95 & 105C the idiot gauge stops reporting the correct temp. Its a designed dead spot in the gauge.

If you want to know the true temp when the idiot gauge hits 90-95C, use the CCU.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)