Howdy All..

I've searched the forum on the topic of the operating temp of a 617 engine, but want to verify that I have this right.

My '85 SD temp gauge lies a few clicks (1/8 of an inch approx.) below the 100 C mark when it is warmed up. In the city and on the highway the temp generally stays at this level.

However, when I climb hills the temp rises to exactly the 100 C mark. Also, at 75mph + speeds, the gauge rises to right below the 100 c mark. When the rpm's drop to "normal" the temp gauge drops right back to its steady state of a few clicks below the 100 C mark.

From what I've read, it seems like this is perfectly acceptable. But, I can't help but think that higher operating temps might be a symptom of less than optimal performance. Any advice or thoughts much appreciated.

What are the outside temps?

Those are slightly higher then normal but still perfectly acceptable. The needle should rest just abouve 80C, at about 85C.

Watch out come summer when you use the AC and idle it around you may have problems.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

With a new radiator installed my 85SD runs about 1/4" above 80C all day long, in the summer with the ACC on.

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The SD runs at 90°C. under a load in the winter. When the load is off and it's just crusing, the temp drops back to 85°C.

100°C. is nothing to worry about. It may also be possible that the gauge is off slightly. If it's not the gauge, then the thermostat is opening a bit late. You might consider changing it before the summer arrives. But, it's not imperative.

Thanks for the replies, much appreciated. I think that I will try and get that temp down though - can't resist the temptation to tinker

My SD with a brand new thermostat runs right at 90c most of the time with a drop below that if I am idling with the heat on.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C