What caused this?

I have similar, if not the same marks on my engine as well, a little over 1,000 miles and estimating about 40% of those miles were with the ICE. 2023 Prius Prime.

 

How do I find exact miles only ICE engine completely? because of oil change interval.

 

You can't. Best you can do is use the %EV.
As for oil changes for the no plug hybrid, just use the total miles. The engine may not actually be running all that time, but the oil will be hot for it. Heat speeds up oxidation and degradation of the oil.

 

Doubly so during warranty period.

A lot here subscribe to cutting the US (10k mile or 12 months, whichever comes first) oil change interval, in half (5k miles or 6 months, etc). Think of it as using the "severe service" interval, if you like.

 

Make sense. Thank you!!!

 

Got a consensus answer: Someone screwed up cleaning the mold, and it's too expensive/not worth it to fix or make a new one.

The mold to make the valve cover is made of metal. Most of it is polished to a mirror finish, but parts of it are textured with something like sandpaper to a matte finish for decoration. The mold is supposed to be cleaned out using proper techniques and cleaners between molding cycles, but either the surface of the mold was damaged with improper techniques at some point or the cleaner isn't doing a good job. What we're seeing is the decorative surface being messed up. It doesn't affect the function of the part at all, so it's probably considered an "acceptable" blemish. I'm betting there's also a thought that valve covers are usually hidden under engine covers. Of course the Prius doesn't have one, but that's more an exception to the rule, and this particular valve cover often does have an engine cover over it in other vehicles.

 

how is that not a factory defect?

 

Because just about every manufacturer of anything makes a distinction between a blemish and a defect.

Does the affected area impact the performance of the part in any way? No.
Does the affected part impact the longevity of the part in any way? No.
Is the affected area noticeable in ordinary usage? No.
Blemish

If the answer to any of those questions was yes: defect.

Every manufacturer is going to have their own levels for what qualifies to make it a defect, but I can't fault Toyota too much for this one. A new mold would likely cost them anywhere from high four figures to low/mid five figures(from my friend who deals with this sort of thing on a daily basis), plus all the affected parts would need to be disposed of. To find this blemish, you have to have the hood open in good lighting and you have to be looking for it. Unless the light hits it in just the right way, you're not going to notice it at first or second glance.

I just wish there was enough room over the valve cover for an engine cover.