Found a procedure for bleeding air out of the cooling system. I performed it but that didn't solve the problem.

I touch all parts of the radiator after the engine had warmed up and the fan was running just to check for any cold spots that would suggest the radiator isn't functioning properly. I couldn't find any.

Then I put a thermometer in the coolant reservoir and the temperature reading was 78C - this, when the car's temperature gauge was showing 100C. That made me wonder, can someone tell me, which coolant temperature does that gauge show? The coolant in the radiator/hoses, or the one in the engine block?

I have another question. After driving the car around town for 25 minutes, I opened the coolant reservoir with the gauge showing 100C and with the radiator fan on and with the engine running, but there was no pressure present in the system. The reservoir cap just opened without any force? Is this normal? The reservoir cap is brand new. I just purchased it from ***************.

Anyone with any other suggestions?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver