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2020 F150 3.5L Ecoboost. Can I fill the coolant reservoir while draining coolant?

The bottom of the radiator has a drain valve and I'm wondering why I can't set up the new coolant to flow into the reservoir while I drain it. I don't mind if I waste a gallon, it would be great if I did since it'd be flushing through.

Essentially using gravity to flush and not let air in.

As far as I can tell, even using a vacuum filler leaves plenty of air since you can't vacuum out air between metal cavities.

Thank you

DDDD
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2 Answers2

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The engine draws coolant from the reservoir based on thermal contraction (as the engine cools), not gravity. The engine therefore must be off to draw coolant from the reservoir. If you run the engine while draining the fluid, the reservoir coolant will not be used and the cooling system will run dry, the engine will run hot.

Follow the engine's shop manual procedures for draining/filling coolant and bleeding the system of air bubbles.

Carguy
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Yes, you can drain while filling. It's quick and doesn't let any air in.

I recommend doing it twice after driving it at full temperature in between to circulate the coolant.

It's so simple, just flushing a couple gallons more often is probably what I'll do.

DDDD
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