What's the cause of this sludge deposit?

 

ups. can you pull all sparkers and post picture of each?

 

Looks to like the plug didnt seal up at some point in time. Had some AF mixture and exaust leak out and deposit.

 

Can't say for sure without more info, but looks like you have oil leaking into the cylinder and out the spark plug hole... bad rings or valve seal(s). Does your car burn a lot of oil? Blow blue smoke? Do you have more pics of surrounding area? Could be as simple as a valve cover leaking above, but can't tell from your picture.

 

I'll have to take spark plug pictures tonight. As I recall, I think they all looked nice and dry. A couple of the plugs may have had some oil on the threads, but not down where they fire.

Here's pics of the engine on the stand with the plugs still in. Look at #4 & #7. The build-up doesn't look like it came down from the valve covers.

 

Blown piston ring?

 

Sounds right to me.

 

As before. Plug picture needed. Could be just that it wasn't tight

 

Here are the plugs. 1-4 are the passenger side, front to back. 5-8 are the drivers side. All 8 plugs are dry where it matters most. 4 and 7 are oily around the threads.

 

I couldn't put all the pictures into one post. Here's plugs 5-8.