changed left rear spark plug passenger side. has a red residue on the base of the plug/not on threads or tip. This cylinder was the one that previously had a head gasket leak and plugged oil flow to the head.. Could this residue be radiator antifreeze ( which is colored a red tone) ??? the spark plug was tight upon removal. engine has 17k miles since heads surfaced/new head bolts/ total 120,000 miles. Any thoughts???? thanks wordman

 

Was this on the ceramic side of the plug? More or less where the coilpack boot would slip over?

Maybe post a photo of the old plug?

Did you replace all the plugs or just that one?

 

The coolant will stain the plug on the cylinder side. If the plugs where changed and it is indeed a red stain from coolant you have a leak and would have at the least a block check run on the cooling system with dye looking for presence of exhaust gas
a brownish ring on the ceramic where it meets the metal nut coil side is a corona ring and nothing to worry about

 

Thanks for your input....the red residue is only on the ceramic part of the plug between where the coilpack boot covered the end of the plug and the cylinder seal ring. ...nothing from the seal ring into the cylinder.... it covered about 1 inch of the plug and all surface. I did change all plugs/ nothing on the other 7. never seen anything like that ...so I am guessing there is a problem. This did not look like a ring..and my first reaction was that it looked like the coolant color. The plugs were in the car for 20,000 miles....If I check the plug in say 2000 miles and see nothing could it be from a bad seal on the old plugl?... Da threw old plug away/ will go find it and post picture...wordman

 

Could it be oil-staining?

A pic would be good!

 

It could be some anti-stick or dielectric compound that the last mechanic smeared on the plug. Remove it and check again in a few months.

 

Ok...I am going to wait and check...It is very likely something could have got put on the plug as that cylinder is hard to get at. Garbage took the old spark plug. Never throw old parts away that you have a concern with !!! Got it...thanks everyone...I f it shows again will have the test done...wordman

 

Finally remembered the phrase ...

It could be "corona discharge" in combination with the red anti-freeze residue.