Need help! Gas engine overheating. Cannot resolve.

I was recently driving down the highway and saw the engine overtemp light come on on my dash. I pulled over and looked under the hood and the coolant was boiling in the reservoir. Bad sign. Coolant was slightly below the low line from evaporating and boiling coolant coming out of the reservoir cap.


I have a 2011 Toyota Prius Level 5 with 77,000 miles with no modifications of any kind.

PROBLEM: Engine coolant overheats up to 235F while driving at high speeds (65mph) under heavy load (going slightly uphill on highway at high RPMs). At low speeds under 60mph or when the car is idle, no overheating occurs.

Sketch-1550725530879 by Sergey K posted Apr 1, 2019 at 3:59 PM

REPAIRS TRIED:

1. Replaced coolant reservoir cap: I started with the most inexpensive part to replaced: the reservoir cap. I got a replacement from Toyota dealer. If the cap is not holding the correct pressure in the reservoir, the coolant can boil faster than necessary. I replaced the cap and filled up the reservoir to the max line. Replacing this made no effect, I had the same overheating problem.

2. Replaced thermostat: Got a new thermostat from Toyota and replaced it. When draining the coolant, it all came out clean, debris free. Checked the old thermostat in boiling water and old one still worked as well. Installed the new thermostat regardless. Replacing this made no effect, I had the same overheating problem.

3. "Bled" the cooling system: Spent about a couple hours running the car up to temp and squeezing all of the coolant hoses. Had the heater on high with lowest fan speed. Doing this made no effect, I had the same overheating problem.

4. Checked the coolant temp sensor: Removed the coolant temp sensor and tested with an ohm meter according to the Prius service manual. Sensor is working as normal. No issues found.

Screenshot_20190319-134352 by Sergey K posted Apr 1, 2019 at 3:59 PM

5. Replaced the radiator: Some friends kept telling me the radiator is probably partially clogged and is likely the issue. Ordered a new radiator and installed it. Again, old coolant came out clean, debris free. Filled new coolant into new radiator and bled the system again. Replacing this made no effect, I had the same overheating problem.

Screenshot_20190321-204436 by Sergey K posted Apr 1, 2019 at 3:59 PM

6. Replaced the water pump: I left the costliest part for last. But my Prius started showing me 1 engine code: P148F: Engine coolant pump over revolution. I'm not sure if this code was here the whole time or not and I'm not sure what it really means. I bought a brand new water pump and installed it, filled system with new coolant, and bled the system. The engine code went away and I'm not seeing it at this time. Replacing the water pump made no effect, I still have the same overheating problem under heavy load at high speeds.

Screenshot_20190322-205152 by Sergey K posted Apr 1, 2019 at 3:59 PM

I checked the service manual and the 3rd gen Prius normal operating temp is 176 to 212F. Mine spikes up to 235F (and would prob go higher if I didn't pull over right away).

Sketch-1553314740704 by Sergey K posted Apr 1, 2019 at 3:59 PM

Some more notes:
1. I dont think it's a blown head gasket because the coolant does not leave the system except when it boiled over. It stays put at the max line now that I pull over before it boils and the oil is clean and not getting mixed with coolant.
2. Both radiator cooling fans work and blow the correct way.
3. The cabin heater works fine and blows very hot air within several minutes.
4. I'm also using the Toyota brand pre-diluted 50/50 pink coolant. Never used anything besides Toyota coolant.
5. No other engine codes or issues come up.
6. When the car is heated up (over 165 or more), the coolant is going around as the left thick coolant hose stays cool and the right thick coolant hose gets fairly warm.


HELP! I've spent so much money and time and literally replaced and/or tested everything I can!!! I need this car for work. I don't know what else to test. It only overheats under load at freeway speeds. If I drive 60mph or less on flat roads, I see no overheating issues and the coolant always stays under 200F.

Am I missing something? Could I still have an air bubble somewhere in my hoses? What can I do???

 

Verified that both radiator fans are blowing correctly. They suck air from the front of the car and blow air into the engine.

The radiator hoses are warm/cool when it sits idle. I have yet to test it when the car overheats.

No, the car will not overheat in maintenance mode. It doesnt even overheat when driving for 30+ mins under 60mph. It only overheats going up hill on the highway at 65mph+. It happens when the car is under heavy load and at very high RPMs.

How can I be sure the coolant system has no air bubbles?

Any other ideas of what it can be?

 

Mendel, I saw that post. Nothing conclusive there. I did a ton of googling and found that that code is only fixed by replacing the water pump.

I replaced my water pump and the code has gone away. I dont get that code anymore, but I do still get the overheating under heavy load at high speeds.

 

Patrick,

-I've owned this car since 2013.
-Yes, its a new occurence. It has never overheated in the past.
-Nothing has happened that may have triggered this that I can think of.
-Its showing 40 to 42 MPG on average which is what it always has got in the past.
- Yes, Im driving in D gear selector.
-When it overheats, I pull over and put the car in park. I open the hood, the engine still runs. The radiator fans are running as well. I wait for it to cool back down and then start driving again. I never paid attention to the speed of the fans. I know they run but I wouldnt know how to tell if its fast or slow.

Tomorrow, I will try the 12V test to each fan and try to compare the fan speeds to an overheating situation as you suggested.

I'll also compare the hose in/out temperatures from the radiator after it overheats.

Im still not sure if there is air pockets / bubbles in the hoses and I dont know how to tell if there is. I have been replacing parts almost every other day and I have a feeling there is air somewhere which isn't letting the coolant circulate properly.

The way I bled the system is:
-Filled reservoir with coolant slowly until the B line.
-Once it held the line, with the reservoir cap still open, I started the car in maintenance mode with heater temp high and heater fan low.
-I squeezed all the coolant hoses by hand one at a time.
-Filled the reservoir again and put the cap on.
-Drove the car on the highway to get over 185F so the thermostat opens.
-Came home, parked, and filled the reservoir again.
-Squeezed all the hoses again by hand.
-Made sure the coolant is at the F level.

Should I be doing something different to bleed the system?

I'll post a reply about the fan test tomorrow.

 

Another thing I noticed is the new water pump housing gets really, really hot when the pump is running, even when car is idle. The old water pump got just as hot. Its so hot that its almost too hot to touch. Is that normal? Maybe no water is getting to it?

 

Sam Spade: The 3rd gens dont have any belts at all. By grill, if you mean the radiator, no its not blocked or clogged and I replaced it. Its brand new and nothing is blocking it.

Mjoo: If it'd be a dirty EGR, wouldn't I see a performance issue and or MPG drop? I'm not seeing that.

TankYoung / Jerry Mildred: the 3rd gen 2011 and up got rid of the bleeder valve. I spent a few hours looking for one. There is not one anywhere. Not on radiator nor the T contraption by the coolant sensor.

Mendel Leisk: Pulling a small hose from the top maybe a good idea. I may try that. Im just worried it will gush coolant everywhere.

I will check the fan speeds and coolant hose temps when it overheats today.

Any other ideas in the meantime? Id love to hear anything.