Haynes Tech Tips - Spark Plugs
Are your spark plugs trying to tell you something?
They can tell you a lot about the condition of your engine. If one or more of your spark plugs have sooty black deposits, that cylinder is probably using too much fuel or misfiring. Oily black deposits are often a sign that oil is getting into the cylinder, usually either past the piston rings or the valve guides.
There’s more spark-plug detective work you can do using the color spark plug chart that’s included in each Haynes model-specific repair manual.