Coolant loss from expansion tank

The car: 2015 Prius Four 127000 miles. All routine maintenance done on schedule. No accidents.

The problem (short story): Coolant loss from engine (not hybrid drive side). It appears that the car is intermittently blowing coolant out of the internal combustion engine coolant expansion tank (what Toyota call the reserve tank) around the cap. I replaced the cap but the problem persists. I see wet and dry coolant spray on the top of the tank, some on the inside of the hood and a lot on the right engine bay to the right of the tank; none around any other cooling system parts.

Long story: I got a check engine light about three weeks ago. I didn't have a code reader at the time. I reset the gas cap and light went out. The a few days later when I was changing the oil I noticed the coolant was so low it just covered the outflow hose. Topped off coolant, bought code reader. Code P0117 engine coolant temp sensor 1 circuit low and P148F engine coolant pump over revolution. I suspect those codes came up due to air in the system from the low coolant and resolved when the air got out. I reset the codes at that point.
Then on two separate days about a week apart I got a very brief overheat light (few seconds) while driving up a hill. On both occasions I got out and noted the coolant was low again. I suspect the overheat light was due to low coolant levels that allowed air into the system (car going uphill, coolant sloshes to the back of the tank and exposes the outflow hose to air, air bubble gets into system, car gets hot until bubble escapes). Topping off coolant resolves issues.

No recurrence of code P148F (pump over rev). Had one recurrence of P0117 (coolant temp) when coolant was low and the overheat light came on when I was going up hill the second time. That second time I got a check engine light with the overheat light. No check engine light the first time.

The problem is not consistent. I can go three or more days without coolant loss/spray then it will spray out sometimes only a little (like half inch) and sometimes spray out most of the reservoir. It does not appear to do this after I drive while car is cooling off, only when driving. I've had no apparent coolant loss overnight.

The other day I had a brief overheat light while driving on level highway. Coolant level was full when I checked it so I finally took it to the dealer and they pressure tested the reservoir, put in on Techstream and couldn't find a problem. The tech took it home that night and drove it for an hour but could not duplicate the issue or find anything else wrong. Radiator fans both operating normally. No leaks anywhere.

My thoughts:
1. coolant reservoir is leaking (but it passed pressure test)
2. cap is bad but that would mean the old and a new OE cap are both bad, so not likely.
3. head gasket leaking? (no odor of combustion gasses in the tank, tank is clean, no bubbling in coolant, car runs fine, no misfire)
4. thermostat is sticking but then the overheat light would be on?

Any help appreciated. I'm trying to not just randomly replace parts.

 

I appreciate your response. I'm curious as to why the pump causes that but will likely give it a try. Seems like the pump is a frequent failure point for the cooling system.

 

I would have thought the Toyota Techstream software would have detected a failure there but maybe not.

 

Thanks for the info. There's no apparent oil level increase. I haven't done the blotter test as I was not familiar with it but will do so. Thanks.

 

I have the same reservations you are but it may be that the coolant is circulating just not at an adequate flow rate if the pump is failing. There are only so many parts to the cooling system and the water pump appears to be a common failure point. I have not yet checked the EGR system or done the blot test but once that's done I will replace the pump and will post here what the outcome is. Since the problem is intermittent it may take a while to be sure it's fully resolved.

How do you know the combustion chambers are clear of coolant, did you scope them? Did you get the codes when the coolant was low in the expansion tank?

 

I am definitely changing the pump and appreciate everyone's help and opinion. This forum has been far more helpful (and cheaper) than the Toyota dealership.

I wonder why the pump doesn't throw out another over rev fault code after the first one. If I'd had that code a second time especially with a full coolant reservoir I would have changed the pump right away. I had assumed I got the P148F code due to air getting into the system from the low reservoir. I did drain some coolant from the radiator early on and watched the coolant get pumped down from the reservoir which is why I didn't think it was a pump issue. With mechanical pumps I assume they are always turning at a constant speed from the belt but with an electric one I suppose it's possible for it to have an intermittent failure? Anyone know more about this? It's interesting that the 2010 model year did not have this code but apparently Toyota became aware of the issue and added it but maybe didn't fix the underlying pump failure problem. Also I've had no coolant venting from the cap for the past two days which makes me think it's intermittent.

This question is purely academic. The pump is getting swapped tonight and thankfully the car seems to still be working fine with no misfires or apparent internal coolant loss.