OK, I pulled the themostat cover off and verified that I had indeed installed it properly. Also starting to rule out air in the system, as the air valve in the thermostat should take care of that.

New theory though...everything fine when driving fast, so outside forced air cooling adequately at speed. Things are fine around town and sitting at stoplights if A/C off. Therefore A/C radiator heat is causing radiator to pull in hot air. This is a problem if the aux fans aren't coming on like they should.

Noticed the aux fans were running when I started the car and left it idling this morning, but it was (relatively) cool this morning (79) and A/C was on.

Everything was fine, normal temps until I got off the highway. Then engine temps creeped up to the 90's. Didn't want coolant boiling over again, so I shut off A/C. When I got to work, lifted hood and aux fans were NOT running at all. Pulled off lead from water pump (I guess it's the aux fan sensor switch) and fans ran, and engine immediately cooled to nominal temps.

Question is, what temp do the aux fans kick on, and did I guess the function of that sensor correctly. Could it be at fault, and how do I test?

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car