Spark Plug Visual Inspection

Normal operation-Brown to grayish-tan with small amounts of white powdery deposits are normal combustion by-products from fuels with additives. Reddish orange deposits can indicate MMT fuel additive which is still used in some markets around the world (will cause misfire and cannot be burned off). MMT fuel additive use should be avoided.
Carbon Fouled-Dry, fluffy black carbon, or soot caused by the following conditions:
- Rich fuel mixtures
  - Leaking fuel injectors
  - Excessive fuel pressure
  - Restricted air filter element
  - Incorrect combustion
- Reduced ignition system voltage output
  - Weak coils
  - Worn ignition wires
  - Incorrect spark plug gap
- Excessive idling or slow speeds under light loads can keep spark plug temperatures so low that normal combustion deposits may not burn off.
Deposit Fouling-Oil, coolant, or additives that include substances such as silicone, very white coating, reduces the spark intensity. Most powdery deposits will not effect spark intensity unless they form into a glazing over the electrode.