Why is my Prius C overheating?

I want to check with the hive to make sure we (myself and the shop) are on the right track to solve an overheating issue on my 2014 Prius C.  As background, the car has 316,000 miles and prior to this it performed flawlessly.  I bought it at 291,000 and the Carfax showed the original owner (who put all the miles on) took meticulous care of it.  Being a Prius it should come as no surprise that the car has no engine temp gauge...anywhere.  So the only way I know I'm having an issue is when the warning light comes on.  The problem only occurs when driving at highway speeds, typically around 70-75mph, after 20-30 minutes.  It'll then give the overheat warning.  As soon as I back off the gas, the light goes off within seconds.  If I get back up to speed, same thing.  No leaks, no burning signs...it's all puking out of the overflow.  First visit to the shop, they pressure tested the cooling system and found nothing.  They checked the block and head gasket to confirm no issues.  Nothing.  They discovered that the engine fan wasn't kicking on, so that was replaced.  Problem solved?  Nope.  Same symptoms.  Again check for leaks, nothing.  Changed the thermostat and radiator cap.  Problem solved?  Nope.  Other clue is that when I'm starting from a stop light I can hear sloshing and gurgling at the firewall (heater).  This time the shop kept it for days and road tested it several times.  They couldn't get it to display symptoms until yesterday.  They took it on a long highway drive and had a scanner plugged in.  After extended highway running at a good clip, the water temp climbed to 250 and the light came on.  Technician backed off the gas and it immediately dropped down to 219 within 30 seconds.  They did another check of the block and head gasket and found nothing.  They are thinking the water pump isn't spinning fast enough to keep up at extended highway speeds...the Prius works hard at 70+.  

Does this sound right to you?  Anything else I should talk to them about?

Your C has a very similar engine to Gen2 Prius...

I have not had to deal with it but Gen2s are notorious for being difficult to get air out of the cooling system.  So, my first thought is air in the system.  Your statement of "gurgle" also makes me think air.

Here is a video for "air in coolant for Gen2.  The logic might translate to Prius C.  I did not find a good video specific to C

Edit: here's a video specific to C coolant (that I didn't watch myself)

Like I said, I have not had to deal with coolant but one thing I remember about these...  Due to the coolant holding tank ("the thermos") bleeding a Prius is not just done by running the car but rather is done by also turning on and off the car multiple time to refill and purge "the thermos" multiple times.  

Link to where to get custom PIDs (Plug In Documentation aka Vehicle specific software) to use with Prius C on the Torque Pro App.  The Torque Pro App only works with android.  But, that can be done cheap enough.  

The app itself will cost you $5 one time

A BT adaptor like this one I have (Panlong) is $13 and could be cheaper with the 20% discount code found in this review.
Amazon US 20% promote code: JD3JHWBE
Amazon US: https://amzn.to/3kSmvY8

If you don't have an Android, visit Walmart and off the shelf buy the cheapest Prepaid Android handset from any Prepaid cell phone company.  There will likely be a $19.99 offering.  It will be crappy but without buying service you can use this unactivated handset like a tablet on wifi.  

$5 + 13 + 20 = $38 and you'll have access to great Prius software.  At it's simplest, that will give you RPM and temp gauge.  Further more it will give you real time voltage readings from each cell of the HV battery 

"Gurgling" in the heater core means air in the system I believe. 

John Welsh said:

Link to where to get custom PIDs (Plug In Documentation aka Vehicle specific software) to use with Prius C on the Torque Pro App.  The Torque Pro App only works with android.  But, that can be done cheap enough.  

The app itself will cost you $5 one time

A BT adaptor like this one I have (Panlong) is $13 and could be cheaper with the 20% discount code found in this review.
Amazon US 20% promote code: JD3JHWBE
Amazon US: https://amzn.to/3kSmvY8

If you don't have an Android, visit Walmart and off the shelf buy the cheapest Prepaid Android handset from any Prepaid cell phone company.  There will likely be a $19.99 offering.  It will be crappy but without buying service you can use this unactivated handset like a tablet on wifi.  

$5 + 13 + 20 = $38 and you'll have access to great Prius software.  At it's simplest, that will give you RPM and temp gauge.  Further more it will give you real time voltage readings from each cell of the HV battery 

Thanks.  I have an Android, let's just say I'm not a fan of (rotten) Apples and leave it at that, LOL.  I will look into that and talk to the shop about it.  I'm leaving to go out of town tonight anyway, not taking the Prius, so I don't need the car.  

I keep thinking air too because of the noise I hear.  Not sure if gurgling is quite right, it really is more like sloshing.  It sounds like water running through pipes.  Perhaps they're not familiar with the Prius C bleeding process.  I'll discuss with them...and make sure they don't charge me for it as I've already paid enough for the repairs done.

The gurgle is definitely air.  Either it's not bleeding properly, or there's air getting in somehow.  I've seen (not on a Prius) a slightly leaky head gasket that would push air into the cooling system under load but didn't lose any coolant. 

A bad water pump is also possible.  Low coolant flow may also make it harder to bleed. 

Torque Pro will also code read and a little more for every OBD2 car you own.  It does not do airbag or ABS codes but for $5 its a huge value!  

I came here just to say it needs more brawndo because its got electrolytes and thats what prius' crave. 

Couple of things come to mind.  With that many miles, have you checked for physical blockage of the radiator-leaves, poplar fuzz, muck?  Also, have you done a combustion gas check on the coolant?  This test identified a leaking head gasket issue I was having.  The coolant overflow reservoir also had black goo in it and it smelled horribly bad.  My car also exhibited the sloshing in the heater core you noted above.   

Does this have a belt driven water pump or an electric one?

I would be sorely curious if the water pump was actually pumping water.  It may be moving coolant strictly by thermosyphon - hot coolant goes up, gets cooled in radiator where it sinks.  You can make about 20hp in some system geometries before you run into overheating issues.  Old motorcycles used this method exclusively, no pump.

Electric water pump is on its way out.  Over time the plastic impeller loses its interference fit on the pump shaft.  It will move enough water to cool the engine at low load/low RPM but can't keep up under higher loads.  This is a fairly common failure mode on higher mileage Toyota hybrids.

Bringing this back up, as I need additional input.  I'm getting really frustrated...mostly with the shop.  They are great guys and I know they are trying, but WTF???  So here's where we are at this point.

All of the following have been replaced.  Thermostat assembly, fan assembly, radiator cap, water pump.  The most recent repair was the water pump, which was about 6 weeks ago.  Shop says they are SURE the water pump was going bad because when they plugged in the scanner, they could see that the impeller wasn't spinning properly.  Pump replaced with OEM.  Next day, overheat light.  Pump alleged to be defective and replaced again with another OEM.  Got the car back and it seemed to finally be OK.  I drove it back and forth to work several times, which is almost 70 miles each way, no problems.  I was able to cruise at 78-80mph, no problem.  I kept my scanner in the car and plugged it in several times and monitored.  Water temp was around 185-195.  It would climb to 205-210 going up hills, which I thought was odd, but it would go right back down immediately.  No overheat light.  I may have heard a little sloshing still, but honestly at this point I forget.  So here I am thinking my problem was fixed.

Last Tuesday, I packed the wife and I up for our trip.  We were flying to Denver, then coming right home and driving straight from the airport to NCM in Kentucky.  Lots of stuff to pack, so as I was loading the car, I took out my scanner figuring all was good.  You know what happens next....  Trip to airport was fine.  First 200 miles of drive to NCM were fine.  Going through mountains in TN, holding steady 70mph or so.  Suddenly overheat light.  Pull over.  See no signs of puking.  Don't want to pull off radiator cap, as it's hot.  Overflow bottle cap was open, but fluid level looked fairly normal.  Oh, I did check all levels before I left and they were fine.  Drop the hood and nurse it the rest of the way to NCM.  I could drive 70-75mph on flat surfaces or downhill, but as soon as I start a slight upgrade, I had to slow down steadily....at times down to 45-50mph.  Get to NCM, and check levels next morning.  I "top it off", which is about a cup of coolant.  Start driving home last night and don't get 30 miles before it starts again.  At this point, the cap isn't super hot so I remove it.  Fluid is a bit low, but I'm thinking there is air in the system as I was now hearing that sloshing/gurgling noise by the firewall.  I spend 15 minutes putting a bit of coolant in, gently squeezing hoses, add a bit of coolant, rinse and repeat.  Probably got another cup to 1.5 cups in doing that.  By the way, this time the overflow tank is full to the top.  Coolant looks fairly nasty.  Oil looks fine.  No other symptoms...no smoke, not running poorly, etc...  No luck.  I spend the next 5 hours nursing it home, at times having to go 35mph up hills.  

What the fresh hell is going on?  Shop insists they smoke tested it twice, pressure tested it twice and the head/gaskets/engine are fine.  They vacuum bled it.  That gurgle noise tells me air, but why?  Seems to have gone away for a little bit, but now is back.  This is a great car, it runs well, looks good, is efficient.  But this cooling issue is driving me insane.

There are two cooling systems (at least in my Gen2 and I expect in your GenC also.)  One water pump for the gas engine...another water pump for the inverter/electric side coolant.  In Gen2's this electric side coolant/water pump is a common replacement item.  

Sample video

Rock Auto for 2013 Prius C water pump will take you to:

Electric side pump

Gas side bump

I have not had Prius water pump issues, personally.  

I have heard that bleeding the air out of the Prius engine is tricky.  Could this be an air in line issue?  What I have read, briefly, implies that bleeding the system is just as easy as letting it run.  The complexity comes from the coolant "thermos" and proper bleeding requires many starts/stops of the engine to also bleed this coolant reserve thermos.  

The "nasty" coolant has me looking at wawazat's question about checking for combustion gases in the coolant.

I know they swear up and down the head gasket is good, but you've got coolant leaving and coolant being degraded. That looks to me a lot like the coolant and the combustion not being kept sufficiently separate.

Agree that the "nasty coolant" comment needs to be addressed. If you're in a good enough mood, post a pic. Either way though, the shop needs to explain that. How did they test for a leaking gasket?

The camera won't capture it well, because I can't get a good shot in the reservoir or the hose. That stain you see is exactly what the coolant looks like.

Yeah the nasty and disappearing coolant both suggest there's a leak between the coolant system and something else in the engine, is there any milkshake under the oil cap? Maybe try running the ICE and shining a bright light through the exhaust at night, that should make it easier to see if it's burning coolant.

From my experience (non-Prius)...

Does the coolant smell burnt?
Is it oily?

yes to either=HG

Took it for a 20 minute drive.  When I left, the overflow tank was at the "Low" mark.  When I got back, it had puked a good bit and this is what I saw.  Had the scanner plugged in.  When climbing a hill, temp got to 240...then I backed totally out of the gas.  Within a minute it had dropped back to 205.  On level ground, it ranged 205-220F

since the water pump has been ruled out, this sounds a lot like a head gasket to me. When I first boosted my 318is, I eventually started to fail the HG with very similar symptoms - fine around town, drove great, no issues, but on the highway or higher load situations, it would get hot and eventually boil over. Of course any time it got into boost it would also puke coolant. Cooling system pressure tested fine, no fluids mixing, but it failed the combustion gas in coolant test.

Does the transmission cooler go into that circuit?

Coolant and oil mixing is not terribly common of a failure mode nowadays, but internal trans coolers definitely can and will fail and pump trans fluid into the cooling system.  This also makes the cooling system overflow.  It can also cause overheating because oil is not a great conductor of heat.

I'll look, but don't think the trans cooler goes in??  Was talking to my friend Sonic and he also thinks it's HG.  Also brought up a good point.  Engine has 322k miles.  While it runs great, it's a E36 M3 ton and head may need a refresh.  Entire used engines are about $300 for one with <100k miles.  Maybe more cost effective for an engine swap.

My Toyota coolant is a pink color, did this start as the same stuff?