For some reason my coolant temperature gauge rises to 119 degrees celsius when driving then drops back to 100 celsius but rarely if ever stays at the normal 82 celsius operating temperature. Yet, I have replaced the radiator, water pump, thermostat, thermostat housing, thermometer, and fan clutch. The hoses attached to the radiator are good condition. The thermostat was checked before in hot water to ensure that is was not defective.
Additionally, my mechanic has an infrared laser and simply points it at any engine part and measures the temperature. We pointed the laser directly at the thermostat housing and thermometer which sits about a centimeter away and the infrared beam reads perfect 80 to 82 celsius. The radiator reads the same. But even while idle the gauge reads over 105 celsius. Now, I just had the head gaskets replaced and 9 valves replaced and after putting the engine back to together the black plastic covering over the center of the straight 6 engine does not screw back properly because the threads inside the head are stripped. The cover comes up about a half an inch when the hood is raised but is kept in place by the plastic air intake that goes over the top of the engine. Any advice would be appreciated...

have you replaced the engine/coolant temp. guage?

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1994|040|W124.036|E500
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Yes, I just replaced the temperature gauge. Mercedes names the thermometer that screws into the water pump the temperature gauge. I suppose the cap goes on the single pin thermometer and the wire that feeds through the electrical plastic covering beside the engine then throught the fire wall and into the instrument board could have a problem as well.

Ohm the sensor unit

OHM 110 = 60C
67 = 80C
38 = 100C

I have a 38 ohm /1% tol. resistor in my tool box that I sub in place of the sender unit.. this will tell if the gauge or sender is the fault without having to warm the coolant/run the engine to the different temps......Radio Shack item

Could you provide a detailed explanation on how you use that apparatus? I wasn't quite following.

The Apparatus.... yeah, I like that ..

If you look at the chart I posted , you notice what the ohms of the temp sender unit should read at specified temps and the gauge should correspond .
So, if you unplug the temp sender unit and substitute a 38 ohm resistor in its place , the gauge should read 100 C.
If the gauge does not read the correct temp, you know you have an out of calibration/faulty gauge.
[ or possible poor connection]

And when the sensor is hooked back up, when the gauge indicates 100C, the sensor should read out at 38 Ohms.