I joined a Prius forum to tap into their knowledge but my post isnt visible until approved which made me remember that while there isn't much that's "sports" about the Prius, it kind of has two motors so maybe that satisfies the Motorsports of my grassroots efforts....
I'll paste the post here and just tldr it at the end:
I’ve been searching and probably just overlooked it but haven’t found much.
a little while ago our 172k mile, always faithful gen ii threw a check engine light. I don’t recall which code (I think it was related to the accelerator pedal) but the common cause was cited as one of the coolant switches/valves. While researching it I learned that the inverter coolant pump also goes bad often. The light would sometimes go off but then come back. I drove it like this for maybe 20 miles.
Since we had never changed the coolant, I went full steam ahead and got a new valve, pump and thermostat and new coolant, all from either the local toyota dealer or from a Toyota dealer online.
I drained, replaced the valve and pump and thermostat, and refilled both engine and inverter coolant. I used the bleeder valves and ended up putting back basically the same amount as what I had pulled out. I didn’t measure exactly but it looks like it’s within half a quart if it’s not the same.
It had been sitting for a month after the engine light came on and before I worked on it. I had it on a battery tender for all but the last week and when I got to it it was dead. So that cleared the codes. I was able to jumpstart it.
I ran it around town for about 10 minutes, no problem and the heater worked great like normal. I figured I was done. The next day, Yesterday, I sent my wife and kid on a 2.5-hour drive. No problems.
Today I took it on the freeway and after about 5 mins the check engine light came back on. After another 5 mins, the res triangle came on. Went off after 10 seconds, came back on for a few seconds about a minute later. It did this a few times before I parked it 3 mins later.
I figured it was overheating a little but because it would come and go I figured it was hot but not ridiculous. Checked the fluids and they all looked normal levels.
drove it home, same thing. I pulled the codes and it is 1150.
Also the heater is cold.
topped it off (it took maybe 1/2 cup at the most), and let it cool. Drove it around town and the heater was working ok. But then went cold and it started with the red triangle again but didn’t show the overheat icon this time.
I have since manually triggered the coolant pump and didn't get any air purged that could tell the heater sometimes starts to get a little warm but goes back to cold
I wonder if I have air bubbles causing it to overheat and a bad pump causing the check engine light??
any other guesses??
tldr:
replaced thermostat, coolant as well as a small valve and motor in the cooling system. All toyota factory parts.
it worked fine for a 2 hour drive, heat worked fine.
now it overheats and the heater blows cold, occasionally briefly blowing a hint of warm.
what could cause this? A stuck thermostat would seem to cause one or the other but not both.
I suspect an air bubble but I've had this thing on different angles, ran it hard, etc. it's my main guess for now, but I'm at a loss.
