Coolant loss from expansion tank

overheating happened twice more since, but this time the whole expansion tank was empty. so i acted to replaced it.
i took out the water pump, checked the impeller assembly and found it was hard to turn compared to the one in the new water pump.
pump was definitely the issue. The coolant temp now stays at 90F +-1 even with 90F Socal weather.

 

I own 4th Gen - 2016 Prius.
when P148F came on in Aug 22 (43K)
dealer choose to replace Exhaust pipe using TSB - T-SB-0135-19- at 45K.
now my engine is overheating emitting same P148F+ P00B111, P011511 AND P319000 codes.
Requires Engine replacement - Head Gasket .

 

I have the same coolant loss problem. 2013 Prius with 236k miles. We have had zero repairs and the only maintenance beyond the standard was cleaning the EGR valve, cooler and intake manifold. Does not burn oil (Mobil 1 @ 10k miles). 51 mpg, straight tire wear. First ever Check Engine Light was code PO117 coolant level. Coolant down to 1/2" of the bottom. Hoses dry, coolant looked like it came from the rear of the reservoir or the cap.
This occurred after going ng up a long hill on the freeway but not going over 65. Filled reservoir and had no problems. I replaced the cap.
A month later, going up the same hill it blew out the coolant again.
I know it could be the pump, leaking reservoir or head gasket.

New cap.

 

At what miles?

 

Its a coolant temperature sensor code, not coolant low. But your engine overheated and is in grave danger. I would take it straight to the dealer. Most likely its a water pump or thermostat.

 

I put on a new set of tires at 169k and shortly after that the mpg went from 52 to 40. It took some research but after a few months at 170 k I determined it might be the ERG system. I did it myself, carefully, and found that nobody knew how to get the carbon out of the cooler, which would not let fluid through. After experimenting I found that soaking the parts in Royal Purple solvent, very cheap from Walmart, did the trick.
A week's worth of work but worth it. The only new part was a gasket kit from Toyota. Mpg was back to 52 instantly.

 

Thank you for the advice. I am going to go ahead and replace the water pump and thermostat. It has a lot of miles so not a surprise.

 

??? Why would you think egr if it happened right after replacing tires?
Certain tired will do that.

Oven cleaner and a pressure washer will clear out the cooler in about 30 minutes.

 

100% on this. Was just having a conversation at the gas pump with another owner who was doing his head gasket due to coolant loss. I’d been wondering about the loss of coolant and what looked like boilover spatter on the firewall near the tank. No codes, but occasionally got the hot coolant flag.
After reading this thread I ordered and installed a new pump. Problem solved: original pump impeller did not turn freely.
Big thanks!

 

I found this video regarding the Prius loosing coolant.

I’m experiencing exactly the same issues that you reporting here. One day, I was driving my 2019 Prius and my daughter heard water bubbles noise coming from the passenger side dashboard. I didn’t think much of it, then one day, I turn on my heat and noticed a cold air is blowing, I open the hood to check coolant reservoir and it was empty. I filled it up and the heater worked fine and blowed warm air. After two weeks, I checked the coolant reservoir agin and it was empty; I filled up again. I haven’t scan the car for any error codes but will soon. I appreciate any suggestions and solutions.

 

You have a coolant leak, and all you did was fill it up? Did you use Toyota coolant?
You know you have a problem, and haven't done anything to find out what it is?

 

Do the same thing the car care nut did to verify if it is leaking into the exhaust through the heat exchanger. It is increasingly common on gen4s.

You are playing with fire by not immediately taking care of a coolant loss/leak in an all aluminum engine. Replacing the engine is the risk.