Inspecting spark plugs. Discoloration means different things.
Cracked spark plug will cause misfire and an engine that idles rough.
This is a plug with 75,000 miles on it that isfiring normally. No large deposits, not discolored . Reddish or green deposits indicate burning coolant. Black, sooty deposits indicate burning oil.
Also check for hairline cracks in the porcelain insulator (1).
Platinum / Iridium tip spark plug inspection
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Inspecting Spark Plugs for Cracks And Wear

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