Overheating going up inclines is it time for a new thermostat?

Did everybody just postulate or has his problem been solved?? I'm having the same problem with my Prius. I just had a reconditioned engine put in with no head gasket problems, so it should be working nominally! I'm still having the same coolant issues after re- replacing the water pump and with a new thermostat and there was a new coolant temperature sensor installed with the engine. It's a 2010 gen3 and ran Beautifully for over 257,000 mi. This is a new development and I'm really tired of trying things that don't work!!! Somebody has to have a clue what is wrong! It's not throwing any codes, only that stupid little red light that comes on while overheating. I do live in Phoenix area but that is not new. It's at the mechanics now and he's as frustrated as I am. Can't figure out why the engine will hover around 100 Celsius not come down quickly when cruising normally. Everything should be working fine but it's not. The coolant is circulating. The fans are working. The thermostat is thermostating! Wth?

 

It’s not a 4th Gen transplant? Any chance of a stop leak product in the coolant? You’ve had it since new? Source for the water pumps?

 

Are the fans blowing the correct way? They "should" be, butttt......
Did you use head gasket pretent sealer in the other engine? If so, you can
bet that the radiator and heater core are clogged. The coolant may be flowing,
but it's likely not coolant is flowing through the radiator.
The radiator could be blocked with dirt and leaf's and who knows what.
There could be an air pocket in the system.
That's a start...

 

How many radiator fins are knocked over flat by years of road debris?

 

Was the new water pump an OE, or the local parts store off the shelf item?

 

Hey All... May 2024... I'm writing mostly to say that I am experiencing similar issues. Overheating / flashing red temperature light / when driving faster (70+) on the interstate in warmer weather (90 F) with the A/C on.

2012 Prius, 175K, driving down to Atlanta from North Carolina having a problem. Pink coolant spewing out of the coolant reservoir past the radiator cap. It threw the P0117 OBD-II: Engine Coolant Temperature Sensor 1 Circuit Low (I believe... but I know it was a coolant temp sensor code) after losing coolant. The fact that the coolant is spewing out indicates a real cooling system issue, not just a temp sensor issue.

On the return trip, when it was cooler, and we drove secondary roads with the windows down, no problems at all. No problems around town. So, in my mind, the cooling system is working; just not well to reject enough heat under the higher loads.

No evidence of head gasket problem (no misfires, no coolant loss except for out the radiator cap, clean oil, and clean coolant).

So, my first thought is a stuck / partially open / failsafe mode on the thermostat. However, I just went ahead and purchased a new Aisin electric water pump as well as the Aisin 180 F thermostat because a water pump sounds like a reasonable item to replace at 175K, and it could account for lower flow and thus less cooling. ((However, I guess if the electric water pump was underperforming, that would probably throw a code also. So, I will just consider this good preventive maintenance, which I really don't do enough of.))

One poster's suggestion to make sure the radiator fins are clean is not a bad idea either.

 

My guess is your thermostat is okay. I think the engine coolant pump is starting to fail. Replace it ASAP! Since you have already purchased the thermostat, you should replace it.

I have read many, many, many threads where the Prius will overheat from a failing engine water pump without triggering a trouble code.

After you replace the pump and drive the car for a week, come back and provide an update.

 

Get on EGR cleaning ASAP. Assuming it’s never been done, head gasket is likely to let go soon.

more info on both in my signature (on a phone turn it landscape to see signature).

 

Thank you Brian. That is good to know that the water pump can fail without throwing codes. That makes me feel better about my decision to purchase the water pump.

I will report back after replacing the water pump and thermostat.

Thank you for the advice Mendel... I believe you provide good and accurate advice. I will try to get on the EGR next. I have helped our son clean his EGR system before on other 3G Prius, so I'll have to do mine. Unfortunately, our son did have a head gasket leak, tried to limp along by adding coolant, didn't change oil, and eventually the oil got back enough that he threw a rod. Tough lesson to learn.

 

Don't discount the temperature sensor either. If the sensor is reading low to actual the ECU won't spin-up the water pump, increase coolant flow rates to compensate for the extra heat being generated. Also check your fans, sometime the electric fan motors will die.

Good Luck

 

That is a good point about a low coolant temperature sensor signal causing the car to “not spin the electric water pump up for cooling”, but I don’t believe that is what happened. I had several “flashing red temperature light” situations and more than one loss of coolant situation before I ever got the coolant temp sensor OBDII code set.

And I also think the concern over the electric radiator fans not working is reasonable, but because I only had “overheat conditions” at highway speed, I don’t think the electric fans should be a factor when traveling 70 mph. The wind should be providing all needed air flow without the fans turning on; but maybe I am wrong.

I would like to report my findings thus far…

I replaced both the electric water pump and the thermostat with Aisin brand, made in Japan, purchased from Rock Auto. My belief at this point is that the electric water pump was the culprit, but that is just a guess based upon this information:

Thermostat: I tested the thermostats, old and new, side by side in a pot on the stove with a good glass food thermometer. While the new one looked slightly different in design (copper actuator head was larger diameter), they performed almost identically. I measured the opening of the actuator every 5 degrees F or so. And while I think the new one may have been slightly more open between maybe 190 to 210 F, maybe it was a millimeter or 2 at the most. Ultimately, both of the opened up about 11 +- 1 mm from cold to 212 F. So, the original thermostat seemed to be okay.

Water Pump: When removed, the original water pump looked fine on the face of it, no apparent physical damage and it spun “freely”.

The biggest difference I noticed between the pumps, old and new, was that the new water pump had very strong “magnetic pole” force every 30 degrees of rotation. The old water pump had the same resistance every 30 degrees; but much weaker. So, it appears that there must be permanent magnets in the motor, and they have weakened substantially in the original water pump, and maybe that is causing lower coolant flow and less cooling.

I put in the new pump and thermostat and ran over 75 mph with 88 F ambient temperature with the windows down for drag, and the air conditioning on max, and I did not have an overheat condition. Nothing definitive here, and time will tell, but I think the cooling system is okay now.

Quick notes on the pump and thermostat replacement…

1. I was able to access all of the water pump (12 mm) and thermostat bolts (10 mm) from the top of the engine. I did not need to remove any underbody fairings.

2. I was able to get all of the water pump bolts with a ¼ inch ratchet and a deep well socket. (At one point during water pump removal, I used a thin flat “serpentine belt” 3/8 inch breaker bar for additional leverage to loosen the bottom most nut).

3. The thermostat studs were rusted (a bit surprising as this is a North Carolina car with “no rust”). So, I had to use PB Blaster / WD40 liberally, and literally loosened and then re-tightened one nut about 30 times, back and forth, loosening up a bit further each time, to avoid breaking off that thin stud. Patience was critical, as it always is. Coated them with anti-seize compound on reassembly

4. The radiator drain cock is visible at the driver’s side bottom of the radiator, through a hole in the plastic underbody fairing. I was able to loosen the yellow plastic drain cock by hand, reaching through a hole that is further back in the fairing.

5. Loosening the oil dipstick tube eased removal of the thermostat.