Coolant does not flow through cooler

Hello,
I'm from Germany and was a long time Prius Gen 2 driver for 17 Years. A few weeks ago I bought a 2016 Prius Gen 4 from a private seller and since then I had trouble with that car. It is the European edition with EHR. The EHR is in leaky and the former owner has disabled it by connecting the heater core output directly to the bypass return pass as shown in YouTube videos. He although fixed the exhaust gas flap in the open position, so the exhaust gas can use the straight way through the EHR. So I think this is a covered up problem but technically it should work. But the car gets too hot when driving with more power. With less power and smooth driving, the heater core cools the engine enough to drive, we have cold weather here through the test drive and the few short rides after the purchase. when driving with more engine power the engine gets too hot and first the water pump fails with "open circuit" message, I changed it and then the second water pump fails again with "open circuit". So I take a closer look and noticed, that the thermostat did not open. I changed it, but the old one works in boiled water the same way as the new one. So even with the new thermostat the coolant runs not through the cooler, even when the water gets nearly 99 degree celsius which is 210 degree Fahrenheit. Much too hot. So I wonder what technical issue could cause the hot water not to open the functional thermostat? There is hot water running through the thermostat bypass and I think this should be enough, but the thick hoses from and out of the cooler stay cold.
I would be grateful for further information for what to do with this car.
A CO2 Test says there is no CO2 in the coolant.
Sorry for the bad English, I tried my best to describe the problem and the experiences I made with this problem. I have access to Techstream and could test and read and clear DTC's. Experience with plain old ice technic is available and I drive Toyota hybrids since 2007, most of them very long and a lot of km's, but never with such problems.
Kind regards,
Bastian Fröba

 

I changed the waterpump a second time, both times there where white deposit like lime deposit on the axle of the impeller, but not so crumble but more like plastic.. So I decided to rinse the water system. I build a hose with a gardena connector and insert a water pump without impeller. Then I let clear water flow through the bypass entry of the thermostat. I blocked the original bypass hose and opened the big engine water outlet hose on the transaxle. so the water rinse only the block and the head. then I connect the water outlet to the cooler again and remove the thermostat main inlet. The water now rinse the block, the head and the cooler. Then I connect the main thermostat inlet again and removed the block of the bypass so the water rinse all the small bypass ways: Two in parallel: 1. EGR-Cooler, EGR-Valve, Throttle Body and 2. Heater Core, EHR. I rinse every path for 20 Minutes. I hope the paths are cleaner now.

 

How can I check the radiator with a IR Temperatur gun? From the front there is the climate chiller and from behind there are the blowers. can I take the blowers away for a test? I will monitor the temperature so I will stop the ice before overheating...

 

So, after a lot of work (something between 40 and 50 workhours), the car runs since round about 1500 km without problems.
I bought the car used, the former owner had problems with coolant in the exhaust. At first he thought, it would be a leaky EHR. He tried to fix it with "stop seal" (Kühlerdicht in Germany), but this did not work. Then a small workshop tried to find the problem, they disabled the EHR, but coolant was still leak in the exhaust. Then they changed the EGR (additional to the disabled EHR) and that stops the loss of coolant. But the "stop seal" was still in the parts. They decided to sell the car and found a stupid buyer. After driving for only 300km the water pump was blocked by the rest of the "stop seal" with "open circuit" failure. I opened the pump, found the plastic and wonder what had happened. Then I found the disabled EHR. Decided to change coolant and water pump and try driving. But I did not bleed all the air out of the coolant system. So the new waterpump get a airbubble and reported "over revolution" and "open circuit" at the same time. I opened it again and found more of the plastics in it. So I made a plan to flush the cooling systems step by step with fresh, clean water for 30 minutes. All three parallel water paths. Then change the pump again, carefully bleed every air out and try driving again. It works without Problems. After 500 km I open the pump again, find only a little plastic, flush the system again with clear water and made everything fine again. Then again round about 1500 km and checked the pump again, it is free from plastics inside. So I think it will be ok now, only the EHR is still blocked.