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Spark Plugs – Inspection
Assessing the condition of a spark plug can be a valuable aid to a technician, and a good
source of information about the engines overall operating condition.
In general, a light brown/grey colour tells you that the spark plug is operating at
optimum temperature and that the engine is in good condition.
Dark colouring, such as heavy wet or dry deposits, can indicate:
• an overly rich condition
• too cold a heat range spark plug
• a possible vacuum leak
• low compression
• overly retarded timing
• too large a spark plug gap.
If the deposits are wet, it can be an indication of:
• a breached head gasket
• poor oil control from the piston rings
• valve train problems
• an extremely rich condition - depending on the actual liquid present at the firing
tip.
Signs of fouling or excessive heat must be traced quickly to prevent further deterioration
of performance and possible engine damage.
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