Spark plug condition after 100K miles, what does it tells about the engine

Question

I pulled these plugs from Toyota Highlander 2011,V6,100K miles. All the front side three plugs look good with normal wear on them and no signs of any oil. The three plugs from the rear side are shown in the picture. It is not clear how these plug threads got oil on them.The oil is dried up and looks shiny as well. The spark plugs have no oil anywhere except the threads. The spark plug tubes are clean and there is no oil on them either.The ceramic portion, the hex bolt portion and metal ring beneath it are all clean. I am not convinced yet if the oil is coming from the tube seals on the valve cover. Can this oil come from the engine side? What does it tell about the engine? There are no codes yet and car runs great. The engine oil level is fine as well. This was a routine maintenance plug change. Any ideas, what is going on with the plugs? These are original Denso Iridium plugs.

Answer 1

If the vacuum port which pulls from the PCV is localized to one side or the other of the engine, it could be the cause of what you're seeing. I'm not sure if your engine does this, but it seems reasonable it might, considering. Some amount of oil/gunk gets pulled through the PCV during normal operation. This is kind of "nature of the beast". If all of the vapors and such are only going to one side of the engine, you'll see a certain amount of the residue only on one side of the engine and therefore only on 1/2 of the plugs. You'd have to trace the vacuum line to see if this is actually the issue.

If this does turn out to be the issue, you'll may want to think about getting a small catch can which intercepts this kind of stuff, yet allows the vapors from the crankcase to pass back into the engine. Yah, you have to empty it once in a while, but it's worth it to help keep the engine running clean/correctly.

Answer 2

With plugs that clean, there is no way it's coming from the engine side. That vehicle is known to have weeping well seals, so common that the seal is included with the valve cover seal kit.