2005 Prius coolant reaches 230-240F when driving on highway

Is the Torque instrument calibrated correctly?
Have you used other measuring instruments for comparison?
A faulty measuring instrument can show the temperature on the surface of Mars and inside a volcano vent at the same time.

 

Yeah what are you using to measure this temperature where are you getting this reading from TIS Toyota software freeze frame data etc? You actually should be able to smell this kind of temperature when you open the hood You don't generally forget it I got four generation twos here I couldn't make that kind of boiling coffee water if I wanted to and I'm in the 100° weather in the southeast United States for five six months of year. I'd have to put a piece of cardboard in front of the radiator for 40 minutes possibly more. And what else is happening at this 2:40 if you whip over on the side of the road or pull back into your driveway and whip the hood up what's happening The two main hoses to your radiator for the engine should be so tight you can't even touch the hose because of the heat and it is harder than a 700C bicycle wheel pumped up for a road race holy Jesus so there must be something seriously wrong if you've done all this other work. If you back the fans on the back of the radiator away from the radiator with this kind of heat going on and you run your hand across the core of the radiator relatively quickly is there a big cool spot in the middle? This is a thing. If so your radiator core is plugged and your radiator fluid is going around that plug the cold spot and the hot areas around the radiator or where the water's trying to get past the plugged area very common You need to find out about this quickly before you burn up your engine I would say you've got a hold up probably from stop leak in the bottom core of the radiator assembly that handles the ice engine this has nothing to do with your inverter or any of that My guess would be the radiator core is plugged jump that out or change that core and probably the problems gone In a shop you could take a water hose and make a fitting and connect it to the radiator hose and a container and see what happens but if you feel the cold spot in the middle of your core in the midst of this then you know where your problem was My buddy at the radiator shop diagnoses this hundreds of times a year in the southeast United States on all kinds of vehicles that need coolant to not overheat.

 

Has anyone ever put any additive into your cooling system (ie. stop leak, gasket sealer)?

 

Don't assume a new radiator cap is a good one. I ordered one from Gates for the wife's '98 accord and when pressure tested it would not open. That combined with another cooling system failure could have been catastrophic. Have it pressure tested. Autozone and other places like that loan out cooling system pressure testers which can be used for this. There was a really steep deposit though, something like $250.

I suppose it is possible the temperature sensor is out of spec and the values you read are wrong. Honestly, that seems like a very unlikely scenario.

Maybe the water pump is not pumping very well? Is the belt still on, not slipping, no coolant seeping out?

None of the cooling hoses kinked or pinched? (Like somebody worked on the car and left a clamp on.)

Presumably the thermostat tested good. If this is one of those cars where the thermostat can be put in wrong (upside down or backwards) verify that it is in the correct orientation.

Following up on Biomed1's comment, what does the coolant smell like? Stop leak smells horrible. If it has a bright yellow/green tint and glows under an ultraviolet light that would be another indicator that there was at some point a coolant leakage problem.

Does the heat work? If the core was leaking they might have tried stop leak, and if that didn't do it, bypassed it altogether.

Both upper and lower hoses get hot?

How long have you had this car? Is the grill blocked? People sometimes do that to get better MPG in the winter, but it would cause problems like you are seeing in the summer.

Look through the grill, is the condenser totally clogged with mud or leaves or something like that? That would also act like a grill block.

Sure you got all the air out of the system? Jacked up the front of the car and ran it with the cap off the radiator for quite a while? Turned the heat on full while doing that? Afterwards it should have needed a little more coolant after driving for a couple of days, as the last few bubbles came out.

Are the hoses to/from the three way valve on correctly? I have a vague recollection of people putting those on wrong and making a hash of the cooling system circulation.

If none of that turns up anything you will probably have to work your way around the cooling system systematically to try and find where the problem is.

 

If that picture is correct, everything above that drain valve to the left should be engine coolant flow. Hopefully someone here has more insight. I've never owned a gen2 or serviced one.
I'm guessing the inverter coolant coils (in and out) are the two lower red caps. That should be the cooler inverter coolant region. Hopefully, If you know someone with a gen2 to verify.

 

I believe the part of the radiator you're interested in this the part with the filler neck that should be for the ice engine so it looks like the upper portion The one that's 160°, that would be engine temperature The 115° that's inverter temperature. So on the part that's 160°, that's the part you want to run your hand across not the lower portion that's a completely separate radiator is it not You can see the two pieces where the lines meet up where they join in the middle but they don't join they're just set on the same bracket so the section that's getting 160° let that heat up hotter then when it's around 200 when you run your hand on the top portion for the ice radiator the part that you're showing 160° now it's 200 now when you run your hand across it is there a section in the middle where it's dead cool like you can rest your hand there then when you move away from the center you need to move your hand very quickly as it's heating up fast or it's hot enough that you move immediately The reason you have that center section that's quite cool or cold is because that part of the radiator is plugged with crap or stop leak or something that somebody poured in the radiator usually it just doesn't happen on its own like it's not the radiator breaking down metal particles and all of that unless it's been run with hard water in it all it's life and no coolant which is highly unlikely so when you are in the full throes of high temperature you want to run your hand not the heat gun your hand across that radiator go from the hot part on the edge right across the middle of the upper radiator when you get into the middle of the upper radiator is it cool? If so that's your problem

 

Use your hand . To feel around the core on wiestion

 

Remove the electric water pump and dissect it. If there's any "crud" in there - that's your answer.
A normal Prius would take about 10 minutes to warm-up, that's with the thermostat in place. Your running without a thermostat, so it should've took at least twice as long to get the car up to temperature, so you may have a flow problem and/or possibly occluded radiator. Does the motor temperature come down a few degrees if you turn on your heater full blast and does it work?

PS, jumping the H2O pump relay only ensures power to the H2O pump, it doesn't mean that it's pushing the correct amount of coolant through the passages! False equivalency test....

 

I would go to Walmart to get that Prestone coolant flush kit, and run it trough your systems a few times. This stuff isn't going to unclog your radiator. Your doing this so you don't clog your replacement radiator. That stuff will knock all that other junk loose in your motor and heating circuit. After your coolant starts coming out clean and debris free, then replace your radiator. Beware, you may clog or your heater coil may already be clogged. Crank the heater all the way up, to ensure full circulation in your entire cooling system. Follow the instructions on the flush kit.

Good Luck....

PS. you don't have to use coolant, just distill water, found in any grocery store. You can use tap water, but that's what your seeing and causing your current issues. That white stuff is calcium deposits or silica from hard water or the wrong coolant type mixed in with the factory coolant. It won't matter after you flush out the system.