Question on coolant temperature

So you intentionally overheated your engine by not using coolant? To check for leaks? And now you want to know if using coolant wouldn't overheat your engine? Hope you got money for new engine...

 

235° F sounds very high, especially for water. Are you sure it was water temperature and not some oxygen sensor temperature or something?

It sounds like something else is a problem. In what conditions did it heat up like that? Were you driving it or was it idling?

 

Not being sarcastic! I'm astonished that you'd knowingly let engine temp heat up to 235' when engine coolant would of prevented it? And maybe water is less viscous than coolant, but not by much. Did you have issues with loss of coolant, or just wanted to test for leaks? Regardless, don't do this again. It's not how you're supposed to test for leaks and odds of premature engine failure have increased significantly because of this incident.

 

Water should cool an engine nearly as well as coolant. The coolant both makes it "anti-freeze" and raises the boiling point as well as lubricates the pump and inhibits rust and corrosion. If it reached 235 °F with water it will probably do the same with coolant, except the coolant will be less likely to boil at that temperature.

Air trapped in the coolant lines can restrict flow, but usually doesn't cut it off, especially through the radiator. If the car was just sitting and idling I have a hard time blaming air on the result.

 

The highest coolant temperature I ever saw when I had my Gen III was 203F.

 

Some racecars have pressurised coolant systems that get this hot, but on a Prius or most ordinary engines with coolant they don't go above boiling. It's actually really hard to even get to 200 degrees. When you're dealing with aluminum engines, it doesn't take much heat above normal to warp a head bad enough that it can't be repaired and has to be be replaced. I've been through this routine before with my Subaru, which no longer made replacement heads and had to give up the whole car and wouldn't wish that kind of awful on anyone.

 

235 °F is definitely on the line of bye bye engine. I'd say try to see if it's ok, but if the OP has a chocolate milkshake for oil, oh boy!

All modern engines have a pressurized coolant system that keeps the coolant from boiling a bit above 212°F.

215 °F or 220 °F should/might be ok. But 235°!

 

Yea, should of made that clear... All cars have pressurization to prevent boiling, but primarily race cars, like an old 1980's Ferrari someone I know has actually operates normally above boiling, not just as a preventative. I remember saying to him, "what's up with your engine temp, that doesn't look good?" He said it's designed to operate at those temps.

 

One thing that changes what used to be common sense is that the proper Toyota coolant is 50-50 premixed. If you first fill the system with water, as you never really get all the liquid back out, it is then hard to get back to the correct 50-50 ratio by adding premixed 50-50 coolant to the water that remains.

Also, if you use a water that has mineral content, unlike what they use in the premix, then you've got that stuff in there.

These days, I think I would just do my best to reconnect all the hoses carefully, expect few or no leaks, fill with SLLC, add pressure with a pressure tester, and check for leaks that way

I'm not sure what the trigger levels in a Gen 2 are, but I know a Gen 3's red-thermometer light will come on at 248 ℉:



Again, that's Gen 3, and that's the temperature where the EHRS sensor will light the red-thermometer light. A Gen 3 also has a cylinder-head sensor, and that can also light the same light. I have never separately tested what temperature from that sensor is needed to light the light. I'm assuming it's the same 248 ℉ until somebody shows it's different.

But yeah, in a normal happy Prius, I rarely see much above 93 ℃ (200-ish ℉).

 

Ok. So, I (almost) never use straight water "to check for leaks". I fill the system with the recommended coolant mixture using a "vacu-fill" device because I've wasted too much of my life trying to bleed/burp air out. I use a handheld pump that adapts to the "radiator" pressure cap port to pressurize the cooling system and check for leaks.

If that's good I start the engine and monitor coolant temperature (ECT sensor) in ECM data with a scantool. I want to see temps rise steadily while the radiator stays cool until it reaches thermostat opening temperature (usually 180° or 195°), then the reading should plateau until the radiator fully heats up.

Temps should climb slowly again until the radiator fan switches on. Some cars turn fans on at 205°, some are 215°, some are 230°.

Straight water is a more effective coolant than a water / antifreeze mix. H²O carries more calories of heat energy than almost anything else. BUT, a 50/50 mix has a higher boiling point, a lower freezing point, and has additives to reduce corrosion and water pump damage.

If your car actually got to 235°, then something is wrong. Air pocket, bad thermostat, bad water pump, something.

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And then there are cars where the fans will come on at some weird Fahrenheit temperature, like 203 ℉. How did they come up with 203 ℉? Well, by being Japanese, and tending to program their ECUs with nice round numbers in Celsius, where that's 95 ℃.

That means it can be easier with our cars, for some anyway, just to remember the nice round original Celsius numbers. Fans on at 95, boiling (of normal unpressurized water) is 100, red light on at 120. (And a Gen 3, with its electric water pump, will shut the engine right down at 105 if the ECM has detected a pump problem. Of course that's not a thing for Gen 2.)

 

Also remember that there's a lot of points you can take a temperature reading from and so each engine is different. You may have 235°F water at the outlet, but it may be 160°F at the inlet, and yet the heads themselves may be 290°F whereas the exhaust valves are over 400°F.

 

What kind of Vacu-fill device do you have? I bought one from Harbor Freight once but the quality of it wasn't so great so I returned it before I even tried it. I'd like to get a low priced one, but it needs to be good quality and easy to use.

This brings up an idea... It would be cool to get thermal cam pics of the heat signature of a healthy Prius engine so people could use that image to compare with their own thermal cam images.

Also am thinking a thermal camera might help show where the source of OP's overheating issue is coming from...