Prius 2010 Mystery - Please Help

This is a long post, so please relax with a cup of tea or coffee.

I bought a 2010 Prius back in May of this year with 259,000 miles. I bought it from the original owner, who loves the car but has decided to get something more functional for her family.

I was told that the car overheats when you drive it past 45 mph. Also, I heard a rough knocking noise when the engine starts up. Besides those two issues, neither she nor I saw anything else wrong with the car.

It wasn’t long until I started seeing white smoke coming from the tailpipe, which smelled like coolant. That told me, in addition to the knocking noise, that there was a possible head gasket leak. Fun times!

But before I even attempted such an invasive job, I thought to try other things.

When I drove the Prius past 45 mph, I would get a “check hybrid system” sign along with the orange triangle. Then, the engine would stall out. I will cut off the car, wait a few minutes, cut the car back on and it will drive until the engine stalls out again with the “check hybrid system” sign.

Two codes pop up: one for misfire and one for P261b, which I understood to be an engine water pump failure.

I replaced the water pump with another one that I got from someone off Facebook. It was never used, fresh in the box, and I don’t remember the brand. But I compared the old water pump with the new. The spinner on the new pump spin freely while the spinner on the old pump was rough, so I figured that the old pump was the issue.

I test drove the car after replacing the pump — and the same thing happen. “Check hybrid system” with P261b code and misfire code.

I assumed that maybe there is a mistake in the code, because I just changed out the water pump.

So, I thought maybe the inverter pump needed changing. I changed that out for a new one. Test drove the Prius. Same thing happened. “Check hybrid system” with misfire code and P261b code.

So, I just surrendered and figure that maybe the head gasket needs to be changed — though I just couldn’t figure out how a head gasket leak would cause a “Check Hybrid System.” So after dragging my behind for 5 months, I finally changed out the head gasket.

In addition, I also:

Replaced thermostat
Replaced PCV
Thoroughly cleaned out EGR system, including the cooler (pain in a butt to do)
Cleaned out the intake manifold, including the four EGR ports
Cleaned out the throttle body

I put everything back together as carefully as possible. Heart racing, palms sweaty, hoping that all the labor of this job would produce fruit. I held my breathe, started the car, and it started out beautifully — then it started knocking again, which I expected that it would as I assumed that the system was “working itself out.”

Then here comes the test drive. I drove it past 45 mph….

“Check hybrid system” with misfire codes and P261b.

I put the car into maintenance mode the next day and let it run. No more white smoke. No more coolant smells from the exhaust. The car did not knock in maintenance mode. But after 10 minutes in maintenance mode, the engine stalled out with “Check Hybrid System.”

So, here is where I am at this moment. I’m about to sell this car, but I really, REALLY don’t want to.

Now, here are some questions:

Is it possible that it’s actually the engine water pump, even after I installed a new one?

Could it be a coolant temperature sensor issue? The fans work like they supposed to. They come on when the engine gets too hot.

I did not change out the spark plugs or coils. Can those things cause this rough, intermittent knocking sound?

There are no other codes popping up. So, I’m reaching out to the experts here! Please help me save my baby before I sell her!