I see your point, myv65. I agree that calculating the energy by using the flow rate and pressure drop would give us a proper number. If we converted that number to Watts, then we could compare it to the electrical power, and this is where I am saying that since these pumps are about 70% efficient, 70% of the electrical power is converted to a movement of the mass (body of water). Wether the coolant returns to the pump or not is irrelevant: the wheel of a car returns to its position, several times per second. A ferris wheel also returns to its position.
As for what happens to the energy (what you refer to as something has to leave the "box"), I would agree that it is friction, and to be clear, we're talking about the moment when the pump is turned off, so the water has potential energy, as it is still spinning in the loop, but it does come to a stop, as the flow restrictions cause the water to slow down, by friction (so yes, heat, but also, and mostly, pressure, aka pressure drop). The coolant comes to a full stop, and there is no more energy (other than potential, from gravity, but that's outside of what we're discussing).
Can we try to apply some numbers to Skulemate's results? Just for kicks... let's do the math.
Skulemate: the pressure is just as important as the flow rate. Flow rate alone cannot give you an idea of the energy involved. Ex: 10 gpm at 10 psi, versus 10 gpm at 100 psi. There is a difference in energy level. You otherwise have it correct.
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