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I thought engine coolant flowed to/thru the heater core(s) the moment the engine is started, regardless of HVAC setting, no?
Great question.
Why must me turn on the heater?
 

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.....to force air over the heater core?🤷‍♂️
Yes, I get that. But as you said, doesn't the coolant flow through the heater core anyway?
 

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You are probably right. Heater control valve functions have been replaced by always-on AC compressors and air diverter doors, so no need to worry about heat setting.

The workshop manual has some interesting guidance on filling the system, and specifically calls out that rear exchanger as a place where air bubbles will accumulate. Meanwhile, on examining the thermostat action, when coolant is below rated thermostat temperature, coolant is routed through the block and therefore the cabin heater loops as soon as the engine is running. This promotes faster and more even block heating, and with faster heat available in the cabin on cold start-up. I'm trying to decide whether there's more flow through the cabin heat exchangers before or after the thermostat opens to the radiator and closes to the block recirculation loop. [scratches head] The thermostat has to be hot/open to clear all the bubbles to the top of the radiator anyway. So sometime in that initial warm-up process with new coolant, it's a Good Idea to rev the warm engine some to push any entrained air to the radiator, where it can escape through the opening for the cap. Once air is purged, top up the radiator with coolant and install the cap with the engine still running. Then fill the reservoir to the Max line and you should be good to go.
 
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