Hi guys,
I just recently had the radiator replaced on my '88 560SEL from a Behr to a Niessens. All is good, except I feel like the car runs just ever so slightly hotter than it did before. Today while on the highway going 75mph and being 105 degrees outside, the needle was not right in the middle or slightly less like usual, but maybe halfway from dead middle to the next line above. Maybe I need a new thermostat? Maybe this is the time to get the one that opens sooner since I live in such a hot climate... Thanks - Mike

Why did you replace the radiator?

Sixto
98 E320s sedan and wagon
02 C320 wagon

It had developed a small leak in the back middle area in front of the fan.

Before replacing the Thermostat, I would check for trapped air in the cooling system. After replacing the Radiator it is easy to get air pockets somewhere in the system. Not yet familiar with the 560SEL (just bought one) but usually there is a bleed valve or plug at the highest point of the cooling system to loosen and allow air to bleed off.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

Might also check your viscous fan clutch. If your engine is hot and you turn off the ignition, the fan should stop immediately. If not, the viscous fan clutch might be going. (A two man test.)

Just replaced mine - 1981 300SD w 279000 miles - and was very easy job. Symptoms were that it ran just a touch hotter than normal per the gauge. So far has resolved the problem. Since I am the original owner, I know exactly where the needle should be on the temp gauge.

Stop overworrying. 105F is extreme heat and of course the engine will run hotter. Thermodynamics, delta T, etc. If you want to replace the thermostat, I would go with the standard 87C setpoint. The important thing is that your fans work as designed: good fan clutch and elec fans kicking in at their setpoints.

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Prost!

The 5.5L M117 uses an 80˚C T-stat. The earlier 5.0L uses 84˚C

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Good catch. I would still just go with standard. On hot days they will all be fully opened giving max heat transfer anyway.

I think the fan ON setpoint is 110C for this one, so that would be my threshold for beginning to worry.

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Prost!

I’ll look at putting an 80C thermostat in soon. I may have just been overly cautious. I mean, there’s nothing super wrong I’d think since it’ll idle with the a/c on at night when it’s 85 degrees out for over 20 minutes and the needle is just above the 80 mark and just sits there. I did the fan test and shut the car off while watching and it stopped pretty quickly, which you can do solo with the hood up looking through the windshield. Now I gotta figure out why my recirc button red led light does not come on when pressed. It’s always something.