High engine coolant temperature is too high

Hey there, so today i was burning it up a hill with the pedal to the floor after about an hour of stop and go traffic, and i had an intermittent light come up on the dash. It took over my screen and had kind of flowing water and a thermometer in red. This came and went as i said when i was driving up the hill. I thought, oh the coolant must be a bit low and is sloshing around.

However the prius C owners manual says that this warning "indicates that the high engine coolant temperature is too high". Which is a bit concerning and more critical than a coolant level. I have noticed that there is pink evaporated coolant residue on the hose next to the expansion / zero pressure overflow tank at the front. (Pictures below). So maybe its boiling?
The coolant is bright pink and appears to be in good condition. No coolant in the oil so thats good and its sitting near the full mark.

Anyone see this before, or could upload a PDF of the relevant troubleshooting section from the dealers manual?

im not sure when the coolant was last changed. I have a record from 2016 and 180k km ago, but the records that came with the car are somewhat incomplete, and its brilliant pink, so i doubt its that old... but who knows!

no codes stored. Scanner has a few temp readings. After 30 minutes off i had ECT = 65*c and IAT = 47*c
assume the first one is engine coolant temp. Second appears to be intake air temperature.

The inverter uses coolant too. Are they two separate systems? or would it complain about engine coolant when it meant inverter coolant?

any help appreciated thanks. I will try and hydrometer the coolant on the weekend. Any other ideas? These new fancy cars set codes for everything. Would it set a code for a sticky thermostat?

 

Just had a run up the hill that caused the problem again with a scanner. I didnt have the problem this time but took the following readings at the highest temperature reading (93 *c)

ECT = 93*c
RPM = 1689
VSS = 66 km/h
TP = 24.3%

weird that my "throttle position" should only be 24% when i have my foot to the floor... someone cut me off so couldnt get it up to 80-90 like i wanted. will try again to reproduce another day.

looks like toyota thermostat unit is 180*f which is 82*c so not crazy temp. I probably just didnt stress it out enough....

EDIT: this Weber guy is also very informative:

 

coolant condition as checked with hydrometer is at the top of the scale. So perfect. has not re occurred. a mystery for now...

and if the radiator is like the one in that very excellent video, it has two compartments and the two systems are separate. Makes sense to me. Should have two drain plugs as well if thats the case, but i haven't done a thorough inspection yet.

 

awesome thanks very much guys for your experience here. no codes yet, so no smoking gun unfortunately. perhaps i will try and stress it to see if i can set a code. Would be cool if it recorded the flow rate over OBD, i will check next time i have a scanner on it.

water pump appears to be at least $300 so not something i just want to throw parts at right now. at least it appears to be on the side of the motor not buried in the engine like most are and is only two bolts! what a dream... hope it doesnt come to that but at least doesnt look too bad.

will update any new developments.

 

Just got around to doing this. Was about $330 CAD all in. I did the water pump and thermostat.

There are two difficult steps. First, you need a wobble socket and a long extension. This video demonstrates that part:

The whole waterpump procedure is there, EXCEPT for getting it out of the engine bay. I had to loosen the oil dipstick tube and pretty much all the electrical connectors to get that sucker out. It comes out at one angle only, and goes in 180% from the way its supposed to be.

The second hard part was with the thermostat. The second thermostat bolt really confused me as it appears inaccessible. Remove all the coolant hoses to the thermostat, and you can sneak a tiny ratchet behind and get the 10mm bolt out. Use your other hand from the water pump side (that is removed) to hold the bolt and centre it.

I used toyota coolant for the refill and used a lisle spill free radiator funnel kit to bleed it. The most important thing there was putting the car into maintenance mode, and then revving the engine till you get hot coolant through the left side engine bay radiator hose (facing engine). After the thermostat opens. Also put the heat on full, but fan on lowest.

After about 20-30 minutes of squeezing hoses, no more air came out and i called it done. Ran the car up and down the hill 4 times that caused the initial problem and seems all good there.

The old pump had no sound when you rotated it, but the new one felt a sort of clicking or almost audible tak when rotated. Felt more engagey.

I will post back if the issue re occurs. I would expect that the summer will cause the issue to occur again if its going to.



Edit: few other thoughts. The bottom bolts on the waterpump needed an impact to loosen them up. Would be difficult with just a ratchet. Hose clamps are really annoying, these type. That took a long time. You will get cooolant everywhere so make sure you have two drainpans for pretty much the entire front of the car.