Waterpump question

The reservoir is just for balancing the thermal change of volume of the cooling fluid.
I would say while heating up the engine, after the thermostat opens at 84°C the big circuit or maybe with only the small circuit too, fluid expands and should enter the reservoir. You should notice that.
At constant temperature I wouldn't expect any flow to see. But maybe I'm not right.
While cooling down the fluid should be sucked back again to engine and radiator.

BTW The cooling pump of the Prius II is driven mechanic by the accessory belt. If the belt isn't slipping/the pulley is rotating, the pump is doing it's job (more or less good).

 

The inverter coolant reservoir (mounted to the side of the inverter) should show some fluid turbulence when the Prius is IG-ON or READY, which indicates the inverter coolant pump is operational.

The engine coolant reservoir (behind the radiator) will not show much activity and that is not indicative of the engine coolant pump being operational. The usual failure mode for the engine coolant pump is that the bearing starts to leak coolant and you will see pinkish or whitish stains on the inside of the coolant pump pulley and in advanced cases, those stains will be sprayed on the underside of the hood pad. Or coolant may have deposited on the air conditioner compressor housing. If you see any indication of coolant leaks, replace the engine coolant pump.

 

Welcome to Prius Chat, good luck with your new Prius.

 

Yes that inverter pump is not working. Pretty common issue. The other poster was confusing it with the engine coolant reservoir so don’t get confused by that. You are on the right track The fluid should look like it’s boiling in there lots of flow activity. The pump is dead.
In Ameerica about 5 years ago there was a service recall campaign where they would replace it for free even if it was working at the time.
Anyway it needs to be replaced. Easy way the dealer does it is to get duckbill Pliars that gently clamps off the hoses so very little fluid loss when you remove the hoses to get the pump out that way you don’t have to dump all the coolant. Just unplug the power plug and unbolt it and bolt in the new one.
Easy way to get to it is to pull the bumper down on thAt side and remove the headlight. It’s wide open then.
Super super easy to pull the bumper out of the way. One bolt and 4 screws.

The reason the car is not unhappy is it’s cold out and the trans and inverter are not heating up.
If you took it for a long drive it would get unhappy and throw a lot of dash lights. It needs a new pump
As soon as you can.

 

Welcome to PriusChat! …as has been mentioned upthread, check the underside of the hood directly above the water-pump (passenger side). My 2009 Gen II had the water-pump replaced a while ago by a previous owner, but I did notice a couple of faint lines of a pinkish deposit a while ago (I think it must have been from that occurrence, since they looked rather old and crusty), and I still check to make sure nothing new is happening there!