How to Read Your Spark Plugs and Tell Good from Bad

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Reading your spark plugs involves removing and examining each. Start with the firing end (electrode and insulator) and note its appearance i.e. look, feel and colour. These are the main conditions you will likely notice and what each means:

  • Good condition: If the firing end is brownish or grayish, the spark plug is in good condition. It can continue to be used.
  • Carbon-fouled: This spark plug has black soot on the firing end. You should replace it. Causes of this include driving for long at low speeds or having a dirty air filter.
  • Oil-fouled: Black oily deposits on the firing end is an indicator of oil leakage into the cylinder. The pistons or valve guides could be worn out. Consult a mechanic to address the issue before replacing the plug.
  • Wet-fouled: This means the firing end of the spark plug is wet from repeated attempts at starting the car (engine flooding). Dry the spark plug and return it.
  • Overheating: The insulator tip and electrode surface at the firing end of the spark plug will have melted. It will look shinny or glossed with blisters. Cause of this is using incorrect spark plug, overheating engine or a lean air fuel mixture. The spark plug should be replaced.
  • Breakage: The electrode has broken off or flattened. Main cause is using incorrect spark plug. Replace it with the right one.
  • Oxidation, erosion and corrosion: The electrode surface will look greenish or yellowish brown due to either oxidation/corrosion or erosion with metal compounds such as Lead.
Reading Spark Plugs

Read: Common Symptoms Of Bad Spark Plugs

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