Mud / Gunk on spark plugs?

I was changing out my spark plugs and they all had this thick mud like substance on them. They were only in for 50,000 miles and they are the iridium plugs. Yup I know they can last 100k but I just change them at 50k. They were a bit loose as well when I was removing them. Engine seemed to run just fine without any issues with those plugs. 253,000 miles on the car. It just smells like used engine oil. Any help would be appreciate and thank you!!

That’s what happens when combustion gasses escape onto the spark plugs.



You appear to be using Bosch plugs. Some owners on Scion forums report problems when using that brand as opposed to NGK or Denso. Switching may solve the problem.

1 Like

Both the Chilton’s and Haynes have color pictures of problem plugs with explanations of what’s wrong to cause the problem you see. I’ve seen it on-line but don’t remember where. ('cause I have it in a book…)

Thanks guys!! Yeah I put in NGK plugs instead of the Bosch ones this time.

I don’t see any major gunk problems there, other than that the loose plug effect mentioned above. The color of the electrode looks ok, light grey. That’s a good thing. One problem you should check for is whether the loose plug allowed it to move back and forth during firings, which could damage the threads in the aluminum (presumably) head. Check the plugs themselves too, for signs their threads are damaged. If so good idea to visit a machine shop and have them clean up the head’s internal threads. Good idea to error on the side of caution. 100 K is too many miles between plug changes imo. I’d only consider that interval on cars where the plugs are very difficult to change. You may have prevented yourself major grief.