The condition of your spark plugs can tell you a lot about what is going on inside your gasoline engine(s). Removing plugs and examining the tips and electrodes can help you diagnose problems and potentially save future repair costs:
When you first remove a spark plug, look for water droplets or rust. Either could indicate that water is getting into the cylinder, maybe caused by a bad head gasket, water in the fuel or, on inboard engines, a bad exhaust manifold or riser.
Take a close look at the tip, or firing nose. If it’s a gray or light beige color then your spark plug is operating at the right temperature, indicating a healthy engine. Dry and soft, sooty black deposits indicate that you are running an excessively rich fuel mixture. A wet, oily coating on the tip may indicate a breached head gasket, or engine oil leaking past worn valve guides or piston rings, allowing oil into the combustion chamber. Tiny specs on the tip, or a fractured insulator may be signs of detonation, an uncontrolled ignition of fuel which could be catastrophic if not repaired.
Finally, check out the electrodes for signs of excessive wear. Rapid wear is frequently a sign of overheated plugs. This, along with misfiring while accelerating and/or difficulty in starting the engine are signs that it’s time to change your spark plugs.
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