Oil leaking onto the spark plug

Hi all,

I've had a 2007 Prius since it was new, and it's been very reliable all these years. It has 137k miles on it, and from what I read here, it's possible to keep it going much longer.

A little while ago I started noticing that it was burning oil. There was never an oil spot on the garage floor, yet I needed to add oil fairly frequently. From what I read online, that's somewhat common, but when the check engine light came on, I took it to a mechanic. He said the same, that it's fairly common for it to burn oil, and a spark plug was fouled with oil, so I got the cylinder misfire code. His take was that there's not much you can do without investing a lot more money.

So, I kept an eye on it, and cylinder 1 is always the one misfiring (none of the others do). The spark plug in cylinder 1 is always fouled with oil, and recently I noticed that the bottom of the ignition coil in cylinder 1 also has oil on it. I figured that maybe the valve cover gasket was failing, so I replaced it. I also bought an endoscope so I can take a peek down there. Even after replacing the gasket and the spark plug again, there's still oil getting in there.

I assume that if it's on the outside of the ignition coil boot, then the oil has to be coming from above, but maybe that's not true. I'm out of ideas, either way. Anything I could be overlooking? Any other thoughts? Thanks!

 

Thanks for the insight!

The gasket I bought was the kind that had the rings for the ignition coil holes attached to the outer part, so yes, those all got replaced and are properly seated.

I don't have a lot of experience working on engines, and I'd love to understand this better. Can you explain how the valve seals, rings, or scored cylinder walls would cause oil to get on the other side of the spark plug? It seems like it would have to force its way through the threads of the spark plug, which may be possible with good compression, but I'm not certain.

I know enough to know that it seems unlikely that an amateur can fix any of the three things you listed there...

 

I suppose that's possible. I bought a Fel-Pro gasket, and they seemed reputable. Also, I made sure to check the torque spec on the valve cover bolts.

I did recently clean the throttle body, and there was not a noticeable amount of oil. I replaced the PCV too, because I had seen some discussion about how a clogged PCV could cause a similar problem.

I have never noticed what I would consider "smoke." It looks like typical exhaust.

This sounds like it's kind of a lost cause. Ouch.

 

Ah, yes, I forgot to mention that I did that. I rotated all of the coils by one cylinder. Didn't make a difference, it still always misfired on cylinder 1.