Changing the prius 3rd gen Engine/assembly.

blown head gasket? how many km on her?

 

How many miles or kilometers on it?

The rust-like build-up is very strange: poor gas quality?

When you're done with the engine rebuild, you should clean the Exhaust Gas Recirculation circuit, and the intake manifold. And consider an oil catch can. Some info:

Nutz About Bolts Prius Maintenance Videos | PriusChat

Two or three other videos linked there too, for the full cleaning of the intake manifold, full EGR clean, and Oil Catch Can install.

Good thread:

EGR & Intake Manifold Clean Results | PriusChat

Another:

Oil Catch Can, Eliminate that knock! | PriusChat

Somewhat tools worth having for the EGR in particular:

E8 Torx socket (mandatory)
E6 Torx socket (optional, but good to have, to remove the throttle body studs from intake manifold)
3/8" ratchet wrench, regular and long handle, flex head, you can never have enough
1/4" ratchet wrench, or 3/8" to 1/4" reducer
Ratchet extensions: you can never have enough
Long needle nose piers, straight and bent tip
Ratcheting 12mm box wrench (optional, but makes disconnection of the EGR cooler from exhaust easier)
Torque wrench (3/8" and 1/4" both good to have)
Floor jack and safety stands (or ramps): basically some method to raise front, if you need to take underpanel off, which you may need to, both for access and to recover dropped items.



Attached is a pdf with revised piston and piston ring part numbers, what Toyota is recommending for excessive oil burning. It also has a part number for a gasket kit (including head gasket), which is a good idea to use.

 

The Positive Crankcase Ventilation system is designed to deal with gas in the crankcase. Decades back this gas was just vented to the atmosphere, along with dripping down onto to the road, any condensate.

Now with more pollution control, the crankcase gas is returned to the intake manifold. The theory is that the gas will pass through the combustion chamber, combust with no ill-effects.

In practice, at least with the 3rd gen iteration of Prius, along with crankcase "gas" there's a fair amount of liquid that condenses in the low point of the intake manifold, and likely a lot of mist that passes through, into the combustion chamber, to be burnt, but not as efficiently/completely as the gas/air mixture the engine was designed for.

It's speculative on my part, but I think the airborne pcv mist, coupled with oil getting past the (older style) piston rings, is causing the accumulation of carbon in the EGR circuit to happen faster.

The EGR circuit has a couple of functions: it reduces emissions, and it keeps combustion chambers lower, by design. However, 3rd gen was the first iteration to have EGR, and owners with say 100K miles or more are reporting significant build-up of carbon, when they inspect and clean the system.

This carbon has a couple of functions (deja vu): it reduces air flow through the EGR, and reduces the effectiveness of the EGR cooler (basically a small radiator, meant to cool the incoming exhaust gasses). Both these actions lead to a combustion chamber running hotter.

Of note too: this style EGR was new with 3rd gen Prius, and judging from owner's experience with carbon clogging: was insufficiently tested. 4th gen has similar system, but with much larger passages, and an input that is further downstream in the exhaust. These changes are actually noted in the sales brochures...

Long-winded answer, but yeah, unless you're intending to repair and sell soon, if you expect to keep the car long term, I would recommend an Oil Catch Can. The OCC is basically spliced into the PCV circuit, so that all the gas needs to flow through it, and it's purpose is to catch (distill) as much as possible of the air-borne mist in that gas, cause it to condense and stay in the bottom of the container, only allow drier gas to pass through. It does need to be periodically emptied. The liquid collect is a strange mix: some oil naturally, but also (and in particular after the winter months in my clime) a watery poultice, cream coloured with all sorts of crud.

Read through those linked threads, lots of info.

 

This is from North American Manual:




And searching for ILSAC GF-5, I find this: http://www.ocsoil.fi/upload/News%20on%20Lubes/ILSAC-GF-5.pdf

I think you're very safe, but maybe someone who really knows can weigh in, lol.