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VIDEO: What Causes Spark Plug Deposits?

If you can’t determine if it is carbon or oil fouling, smell the plug. This video is sponsored by Autolite.


A spark plug has a glossy black coating on the insulator of the spark plug. The cause of these deposits could be caused by any of the following except:
A) Worn piston rings
B) Stuck open EVAP purge valve
C) PCV Valve Stuck Open
D) Worn Valve Seals

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The answer is B. The deposits are caused by excessive oil in the combustion chamber. If the Evap purge valve was open, it would typically cause a Fuel lean or rich condition. Oil fouling of a spark plug typically results in shiny black appearance. If enough oil is in the combustion chamber, the deposits can build up on the tip, porcelain or shell. If you can’t determine if it is carbon or oil fouling, smell the plug; it will smell like engine oil. The oil can come from the piston rings or valve stem seals. Leaking piston rings can be diagnosed with a leak-down test. If one cylinder has oil fouling, a relative compression check can help to assess mechanical issues with that cylinder.

Valve stem seals are more difficult to diagnose. Smoke from the tail pipe during a cold start, long idle period or high engine vacuum events could be a symptom. Malfunctioning PCV systems are becoming a leading cause of oil fouling for modern engines. These systems have become more than just a spring-loaded check valve. Modern systems can separate oil from the crankcase vapors and electronically regulate when the engine ingests the vapors. If the PCV valve is stuck open, the excess vapors and oil droplets can quickly foul the spark plugs. So just remember shiny insulator are caused by oil, matte insulators are cause by fuel when you take an ASE test or you are in the bay. This video is sponsored by Autolite.

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