It seems fairly easy for the OP to get from cold start to over 100*C. Get to that point then pop to hood. You should see the aux fans turning full tilt and hear the blast from the clutch fan. A fully engaged clutch fan will drown the clatter of a 606 at idle. Check that the upper hose and radiator are hot indicating the thermostat is open. The clutch fan needs a hot radiator to engage. Go for a drive to get the engine below 90*C then see if subsequent prolonged idling sends temps over 100*C again. My money would be on clutch fan if it gets that’s hot again. Thermostat if it gets hot once then not again.

If the thermostat and clutch are originals, change them for the heck of it.

Sixto
98 E320s sedan and wagon