Over-rich fuel condition: This is generally indicated by black and fluffy soot on the insulator. It can indicate a leaking injector or an out-of-adjustment carburetor. This can cause low fuel economy, decreased performance, and even damage to the catalytic converter.
Partially melted center electrode: Can be caused by the ignition being too far advanced, combustion deposits in the combustion chamber, a defective distributor, incorrect air/fuel mixture, or poor-quality fuel. Can result in misfiring, loss of power, and insulator-nose breakage.
Heavy wear on the ground electrode: This can be caused by fuel and oil additives or poor flow conditions in the combustion chamber due to combustion deposits. Because the ground electrode has worn, the gap is far too large and the ignition voltage is no longer sufficient for the large electrode gap. This can cause misfiring and poor starting.
Side electrode bent or moved: The plug contacted either the valve or piston. Using the wrong plug generally causes this. Always use the plug recommended for your vehicle.
Oil-fouled: Oil is entering the combustion chamber. This can manifest itself as either an oil-covered face or by white deposits on the insulator. Causes can be scored cylinder walls, badly worn piston rings, and/or worn valve guides and seals.
Problem: Carbon fouled Marked by a soft, sooty carbon deposit. If all of the plugs are car
Problem: Thermal Overload Matt Hallis, Product and Technical Support Manager at Bosch Corp
Problem: Carbon flashoverThe folks at Bosch tell us that when this happens, the spark can