I watched the needle again today as I ate lunch in my car. When idling in 102 degree heat, the needle hovers around 100. It goes just a hair above 100, then I hear the aux fan kick on, and the needle drops to just below 100 and stays there, with the aux fan running the whole time.

The radiator is free of bugs and is squeaky clean both inside and out. It is not the original radiator but was upgraded to the newer type with metal inserts. Either way, this experience has scared me to action. Among other things, I have ordered a new heater hose and thermostat from FastLane and will be installing them as a precaution. I will also flush everything and refill with the correct zerex coolant.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280