I use bimmerlink to monitor coolant pump speed and fan speed as well as oil temp and coolant temp.

You need to know the actual temp to diagnose this.

You are describing the symptom of a weak coolant pump. Say if the pump is turning 2/3 speed the extra radiator aka heater core is pulling just enough heat to keep things under control.

Thermostat not opening properly, coolant pump not running full speed are the prime suspects.

You can at least monitor coolant temp from the hidden menu for starters.

I recommend getting bimmerlink

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)