Spark plug engine coil number 4 had oil on it when it removed it!

could be rings/valves. do a compression test

 

My experience with this symptom is that it was due to a leaking valve cover gasket, which is an easy and affordable fix... And most would recommend replacing the PCV valve while you in their working on it.

 

Yeah, could be this:

 

Interesting... So many needless changes between Gen2 and Gen3 Prius... In Gen2 the gaskets for the spark plugs are attached to valve cover gasket. Of course they also changed the name from valve cover gasket to cylinder head cover gasket. So stupid! Early model Gen3 changed so many things to eliminate Prius simplicity, durability and ease of repair. Every day I seem to find another example of that.

 

That picture, with the spark plug torque spec, is maybe a thousand pages away from the rest of the spark plug info, in the 3rd gen Repair Manual.

 

I'm not sure I can lump the cylinder head changes in with that ... consider the old-generation's select-fit lifters for valve adjustment (intake and exhaust different, 35 part numbers each) making DIY valve adjustment effectively a non-possibility; swapping the engine is less fuss. Gen 3 goes to very DIY-friendly hydraulic self-adjusters (one part number!) (and adds roller rockers to boot). Almost by itself that's worth the price of admission.

If in exchange I have to learn a different name for a gasket, well, maybe it builds character.

 

I used to feel right at home, with Honda screw and lock nut valve adjustment. Except for our last, 06 Civic Hybrid, which required cowl removal, and then another hour or two coaxing a wiring harness off the top of the valve cover.

 

I was really quick at valve adjustments on my '72 Mitsubishi. Carb and point adjustments too. You get that way with things you have to do multiple times a year.

All the same, I can't say I mind not having had to do any of that for the last quarter century or so.

 

This valve cover is (was) manufactured in millions. Rest assured they have thought it through before creating separate gaskets.. Sometimes it’s better to have multiple simple parts instead of one overly complicated, expensive and difficult to manufacture and assemble part. Afterall, separate spark plug gasket is the normal way. At least what i have seen.

 

Or as Elon Musk says, "sometimes the best part is no part." He explains that a huge amount of engineering and design time is wasted in optimizing a part that shouldn't exist in the first place. For example his landing legs on starship. Rather than build and optimize landing leg design, he eliminated all the legs and instead built a 440 foot tall tower with enormous chopsticks capable of lifting 100 tons to "catch" a landing starship. There's no proof this is a reliable solution yet, but pretty impressive if it is...