2018 Toyota Prius 4 coolant leaking, water sloshing and strange noise in coolant reservoir

Your description sounds very similar to what I experienced with a Volvo I used to own. In that case, it was a blown head gasket. During the compression phase of the cycle, combustion gases are pushed past the failed portion of the head gasket, allowing said gases to push into the cooling system, causing overflow. It would make sense to me that removing the coolant cap o-ring alleviates the symptoms, as it provides a pressure release pathway.

With a head gasket leak, you’ll often notice coolant in the oil as well. Once the engine is turned off, the residual pressure in the cooling system will often push coolant the opposite direction and into the combustion chamber chamber (cylinder). You’ll then see a milky apprearance to your engine oil which is at that point mixed with coolant.

This all being said, hard to diagnose a car over the internet, so this is just what occurs to me to be likely based on your description. I don’t recall the 4th gen Prius being particularly known for head gasket failure like some of the prior gens were. Usually on a 4th gen, if you start losing coolant, it is the heat exchanger under the car where the exhaust gases heat the engine coolant up to operating temp more quickly. If I recall, the 4th gen’s are somewhat known for this failure point. I would venture a guess that this is not what you have going on due to the fact that the heat exchanger failure would likely not result in your overflow situation, since there wouldn’t be pressure introduced into the cooing system at that point. You’d just be experiencing coolant loss.

If I was a gambler, which I’m not, I’d bet on a head gasket issue purely based on your description of events, which is pretty classic head gasket leak. The water pump might be making noise due to gas being in the system, so you have some cavitation going on.

Just my 2c. Maybe someone more in the know than myself will chime in with better info.

 

It is time for a replacement engine!

How many miles on the car?

 

removing the o-ring wasn't a fix - it was a bandaid to an existing issue. Did you think OEMs simply place unneeded parts on a car?
Brian's correct, time for a new engine.

sorry......

 

Exhaust Heat Recover system in the earlier 4th gen years (till 2019 model year?) have been prone to start leaking coolant into the exhaust. That can be misdiagnosed as head gasket failure, or: if it drags on long enough, engine runs dry and it escalates to head gasket failure as well.

The EHR system is integrated with catalytic converter (one part), and very pricey to replace, and chronically in short supply. There's a warranty extension bulletin from Toyota, but between the limits of that warranty, the short supply (perhaps on purpose by Toyota) and dealership's "amnesia" regarding that warranty (perhaps because customers pay better than Toyota?), getting it fixed in a timely fashion can be like pulling teeth.

There is a workaround, that dealerships won't do (defeats pollution control), to simply bypass the EHR system.

 

OP stated that he had an engine oil to coolant cross-leak that he thought he rectified by removing the O-ring - preventing the coolant system from pressurizing. Likely didn't dive into it to find the reason of the actual leak - most likely head gasket between coolant jacket and oil channel. There's a high likelihood the head and/or engine mating surfaces are warped by now.
The gurgling and noises are hot gases - water turning into steam hitting the hot surfaces of the engine parts and evaporating immediately causing air pockets in your cooling system. Your cooling system capacity is only 1.75 gallons; so if you put in a gallon - your cooling system was 2/3 empty.

 

Here is the youtube link:
https://youtube.com/shorts/UwfK-Qt17sI?si=5e_FMYijzX0l53RL

 

Yes, I bypassed the EHR, but it’s still leaking. The coolant was getting into the engine oil because of high pressure. After I removed the O-ring from the reservoir cap, the coolant stopped mixing with the oil. Now I suspect the coolant is overflowing from the reservoir due to high pressure, but I’m not sure why.
Here is the link:
https://youtube.com/shorts/UwfK-Qt17sI?si=5e_FMYijzX0l53RL

 

Do you think it could be water pump going bad. i heard the strange sound. here is the link:
https://youtube.com/shorts/UwfK-Qt17sI?si=5e_FMYijzX0l53RL

 

This is very confusing. It appears that there are now two discussions that are happening in two different threads by the same person using two different usernames on this forum. It appears that users "dv.1000" and "Willow.D" are the same person.

The link to the other thread is here, https://priuschat.com/index.php?posts/3584349

 

yes, very

 

FWIW; the car has also changed - one's a gen4 the other is a gen3 according to profiles. The only consistency is the state of Texas.
Willow.D has been around since 2019 and should know better than to high-jack a thread.
I'm just going to shut-up till there's some transparency.