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Gauge Engine Health by Spark Plug Condition

The color and condition of a spark plug can reveal much about what is going on inside your engine. For example, a plug coming from a properly operating engine is grayish-tan to white. Don’t be alarmed if the plug is pinkish-red because this comes from using additives in unleaded fuel. Appearances that warn of engine problems include:

  • An insulator tinted charcoal with a firing tip that is damp with gas indicates a faulty choke, overly rich fuel mixture, ignition problems, leaking fuel injectors, or too low a plug heat range.
  • A firing tip covered with black soot can indicate too cold a plug heat range, an improperly adjusted carburetor, or malfunctioning choke.
  • When the ceramic tip, center, and electrodes are coated with a black, oily substance, then oil is entering the combustion chamber, indicating worn rings, valve guides, or valve seals (shown at right).
  • A cracked, chipped, or broken insulator is caused by low-octane fuel or over advanced timing.
  • Electrodes that are eroded, have rounded edges, and are excessively worn away should be replaced.

Tips to tightening spark plugs

There is definitely a trick to properly tightening spark plugs. Autolite offers this guide to follow in the absence of a torque wrench.

  • For 14-mm and 18-mm spark plugs with tapered seats, tighten the plug 1⁄16 turn beyond finger tight.
  • Turn 14-mm and 18-mm gasket seat plugs 1⁄2 turn past finger tight.
  • Turn 12-mm gasket seat plugs 3⁄8 turn past finger tight.
  • Turn 10-mm gasket seat plugs 1⁄4 turn past finger tight.

Always install spark plugs with clean and dry threads to avoid stretching or overtorquing the plugs, which can result in engine damage.

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