Oil marks on dipstick

I'm NOT using that crap. It's not worth the time. And it doesn't work.

There's no point in changing the oil if you don't replace the filter also...

 

To answer the question of when you open the coolant cap and you hear hissing or pressure.


Yes that is a clear indicator that many do not know.

After driving and letting it sit overnight
a prius without a headgasket issue will not make a hissing or pressure sound at a cold start situtation.

A prius with a headgasket issue will however still make a hissing or pressure sound when taking off the coolant cap.

Normally a hot-warm engine will do the same, so a lot of people mistake this for that but it should never do it when cold.

 

To answer the question of when you open the coolant cap and you hear hissing or pressure.


Yes that is a clear indicator that many do not know.

After driving and letting it sit overnight
a prius without a headgasket issue will not make a hissing or pressure sound at a cold start situtation.

A prius with a headgasket issue will however still make a hissing or pressure sound when taking off the coolant cap.

Normally a hot-warm engine will do the same, so a lot of people mistake this for that but it should never do it when cold.

 

Mine does that. I guess that explains why about 5 fluid ounces (~150 ml) of coolant disappears every two years.

 

It's very likely you have a leak somewhere. A pressure test should locate it.

You may lose an ounce or two between the 50,000 mile changes as the water in the coolant
slowly evaporates from heating and cooling.

 

"2014 Prius, 360,000 miles, head gasket replaced 110,000 miles ago"

It sounds like you have an advanced head gasket leak now and somehow missed the early stages this time - perhaps because the earlier hg job used a better gasket.

 

360,000?
MILES?
That's further than the Artemis mission is set to fly away from the earth.....
BY OVER 100,000 miles!
That's nearly 15 laps around the planet.

SO....
No deets about how long you've had this particular G3 but even if you bought it new you're still knocking down about 30k miles a year.....
....for now.
You have the automotive equivalent of a centenarian that still goes to work everyday and mows their own lawn.
What type of oil you use is like arguing over the food pyramid.
There are two ways to diagnose a failing head gasket.
Go to an auto parts store and get a leak-down tester (they will usually let you borrow one.)
Or?
Find a gun-nut in your neighborhood that has a bore-scope....which in Lexington, Ky ought to be about as hard as finding an ICE protester in Minneapolis.

I have no bleeping idea what to tell you after that.
If you're handy with tools, you can re-gasket the car for a few hundred dollars in parts and (according to the Googles) about 20 hours - for a mechanic, or somewhere between 4000-5000 dollars in labour costs.
Maybe a little less if you know an independent wrench that owes you a favor.
No idea what this all cost you a quarter million miles ago or how you did it.

I'm thinking that the problem wasn't the head gasket to begin with.
Remember that a head gasket is like a fuse.
Its job is to keep the engine pieces-parts inside the engine.
The analogy isn't perfect because you can still introduce enough coolant to a cylinder bore to hydro-lock the engine after the head gasket fails....or because it failed.
It's a chicken and egg thing.
The Prius Death Rattle is your warning for that.

"Where would it all go?" is an excellent question!
If you do not have a chocolate milk shake in your oil sump then it's not all going there.

That's why the first place I would look is in the cylinder bores.
One of them might be getting 'steam cleaned.'


Personally I would try to find another car.
Quickly.
I might consider a low mileage G3 and using the one you have as a parts barn but low mileage G3s are about as easy to find as a MAGA hat in Minneapolis.

Face it.
It might be time to consider seeing other cars.

Good Luck!
Stay warm.