How common is Gen 2 Head Gasket Failure after 300,000 miles?

Cliff's notes:
Even after head gasket replacement, coolant is still mixing into oil (but not into cylinders as far as I can tell and not oil into coolant). Coolant does not even have to be under pressure, but engine has to be running.
Failure is either head gasket or a block crack or head crack that opens only when engine is running.
Guesses?

 

I don't understand.

 

Yeah, sounds like a plan. I'm thinking about picking up a used engine, but that could, of course, come with its own set of problems.

 

I did the balloon test, Trojan Magnum, on both the radiator cap opening and the overfill tank opening, and no pressure at all leaking to the cooling system side. I did not test with color changing fluid because I didn't already have some and after the balloon test there seemed to be no point to do so.

I would be too scared that with my luck I would just end up plugging up the radiator with any "mechanic in a bottle" stuff.

The replacement engine now has 320k miles? How many did it have when you bought it?

 

where do people usually source the kit? Seems like rockauto does not sell a complete kit, or maybe I'm just blind again. About how much $ is a decent price with pistons and without?

 

I didn't have any overheating that I was aware of during the 60k miles I have had it. The only unusual thing I did have was intermittent misfire due to one defective NGK plug from rockauto.
Yep, in my case dropping in a new gasket didn't work. I checked the head in various directions and could not fit a 0.003" feeler gauge under the ruler anywhere. (I think that one is actally suppoded to use a 0.002", but I don't think that would have cleared either).
I am going to take the head to a machine shop for resurfacing since they claimed that it would be "about $60-70" and take a lot closer look at the block deck (looked good) before proceeding.
My cooling system holds pressure for days and days when the engine isn't running. But, as soon as it is, coolant starts entering oil (coolant isn't even warm yet, and definitely not under any pressure yet). I'm worried that I have a cracked head or block.

 

The block does not have last x digits of VIN, right? So no true way to verify a "Only 49k miles" story about a used engine.

 

1. I was asking for educated "guesses" on how common cracked heads and cracked heads are on Gen 2...
As stated previously, when the engine is not running, the cooling system holds pressure for days and days (I'm testing at 15psi so little more than the13psi rating of my radiator cap). That, of course, is a situation much more likely to reveal a leak than just water pump running on a cold engine.

2. I'm in the US. I usually use Rockauto for new parts.

3. Makes very little sense? How? The "chocolate drink" just would not separate. Synthetic oil (and Toyota coolant). Maybe conventional oil could behave differently, I would not know.

4. I thought about confirming good oil pressure before putting in the time to change the head gasket, but since I already had the head gasket and bolts, I decided to not worry about measuring oil pressure.

5. The engine runs fine, but coolant is still mixing into oil.

6. Yes, purchase of a newer car is in the future, but probably not until later. I have other vehicles, but the Prius fits a particular niche as my other houses and storage building are about a 200 mile round trip from my current primary residence and my other vehicles are lucky to get 10 mpg probably.

The only used engine I am likely to buy will be from a seller that will let me 1) put my borescope in each cylinder 2) remove and cut open oil filter (and replace with a new filter, if they wish), 3) do a leak-down test, 4) remove valve cover and 5) remove (and retorque) cam caps.

I think this will limit me to a subset of private sellers and small yards. Cost will also limit me to these sellers (and impossible to reliably verify mileage engines, unless they happen to still have engine in a car they are parting out) because the car was $1400 several years ago and worth less now (even if the head gasket was good).