Car: 1991 560 SEL 103K miles
Problem: temp gauge never go above the tick between 40 and 80 C even after 30 minutes of stop and go.
1st Diagnostic: Thermostat stuck open.

Spend three hours to get that ^&*^%$ thrid screw to get the T-stat out. Put the unit in a pot and cooked it. The 1st stage open at 82 C and 2nd stage open at 95 C, working perfectly.

Now what? Please advise how to check the temp sending unit?

Low engine temp seems to contribute to my car running high idle. Engine keep wanting to go to a higher RPM to get warm.

You could verify the engine temp if you have access to an infrared thermometer.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

It's just my opinion, but the cooking-on-the stove thing is not proof conclusive.

Buy a new t-stat and make sure it's a Behr.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I'm sure that your temperature gauge does not show the correct temperature. Engine combustion produces high temperatures which have to be cool. Just drive the car for 10 minutes, open the hood and touch the upper hose and if you can not hold it the temperature of the coolent is higher than 40-80C. I will look for the problem in the sending temperature sensor or wiring from the sensor to the gauge.

Roman
87 300SDL 152K
86 300SDL 212K
86 420SEL 226K
81 300D 139K

I did exactly that when I got home Thursday night (30 minutes drive). I could put my hand on the radiator feeling hot but not burn.

I have a behr T-stat coming Monday and we will see what happens after put it in.

air lock run the engine without the rad cap on so all the air gets out and watch for the cooling fan to cut in then all is ok