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Old 08-06-2005, 11:16 AM   #1
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1995 SL2
Information Spark Plug "Corona Stain"

Corona Stain:

A brown stain - often mistaken for gas leakage - is sometimes observed in a ring around where the insulator joins the metal shell of the spark plug. This discolouration is generally called Corona Stain and is the result of oil particles present in the air around the engine adhering to the insulator surface.

The Corona Stain is often seen on spark plugs that are installed deep into the engine such as on many four valves per cyclinder engine applications. This is because oil particles are not blown away by air circulating around the engine compartment and thus adhere to the insulator surface. It is imporant to understand that Corona Stain causes no deterioration to the function of the spark plug.
> 95 SL 2 = 653,369 Miles 40.4 MPG, as of 5/20/11. My manual Radiator Fan Switch, courtesy of Wolfman's patient installation guidance, continues to be.......invaluable < The car was retired
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Old 09-26-2005, 12:48 PM   #2
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Default Re: Spark Plug "Corona Stain"

I guess that's a good indicator of a leaking cam cover gasket, even before it gets out-of-hand. Good topic, Luke, I always thought it was just heat discoloration!
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Old 11-07-2005, 07:47 AM   #3
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1999 SL2
Default Re: Spark Plug "Corona Stain"


I know this is a couple months after your post, but thanks for posting these pictures. I just changed out the plugs on my wife's 99 SL2 yesterday and this is EXACTLY how all four of the plugs looked. Additionally, I noticed that the threads on the cylinder end of the plug each had a thin film of oil on them.

I'm going to proceed on the assumption that the cam cover gasket is beginning to leak even though there was no real accumulation of oil in the plug wells and the plug wire boots were clean. It seems that if there is indeed a leak, it's fairly recent and not all that profuse at the moment. Better to catch it early in the process and nip it in the bud.

Thanks for the link.
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