Air pocket in the cooling system

welcome! did you change the coolant? have you tried cycling the heater core on and off? you might need tech stream, but there are some threads here with good advice. all the best!

 

Odometer reading?
Have you tried replacing the radiator cap?

 

Some possibilities - none of these have to be visible to let some air into the system.

• Head gasket problem
  
• heater or radiator hose or connection
  
• radiator leak
  
• overflow tank leak

 

Yes, a leaking cylinder head gasket might allow excess pressure from the combustion chambers into the radiator.

The OP should have a cylinder leak down test performed to rule out that possibility.

 

After driving for a while feel the upper than the lower radiator hose. They should be the exact same temp. If one is cooler than the other get this issue fixed asap as your not circulating the coolant. Not circulating coolant=engine overheat=-blown head gasket.

Turn on the cars heat after warmup and see if you have heat. No heat your def not circulating coolant.

Does the car have its fan belt installed and is it tight? Its only job is to turn the water pump. Most likely air pocket in the CHRS tank.
Or water pump is shot.
Wish you came on here and asked about this as I would have recommended not to change the coolant but just change the rad. Just dump the rad only. Do that once then again in a month. It really helps and will not invoke air into the system. Prius is extremely easy on engine coolant.
Conversely Prius is very hard on the Inverter coolant and that requires much more attention and much better bang for your DIY dollar.
Very easy too much easier than the ICE coolant.

 

We have different understandings of what a radiator does. The whole idea of a radiator is to radiate heat, cooling the engine to the atmosphere. If the thermostat is open and the radiator is not clogged, there should be a temperature difference between the two hoses. If the thermostat is not open (or the radiator is severely clogged), there's no flow and the hose temps would be equal. Equal temps could be normal on a cool and idling engine, but it's not always a good thing!

Here's a good way to check cooling system performance with an infrared thermometer. It mentions that a properly performing radiator should have a temperature differential from hose to hose.