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Spark Plug Fouling by Dan Coles

Spark plug fouling in your aircraft engine may be a problem. In many cases, spark plug fouling results from the tetraethyl lead (TEL) in aviation fuel that can be reduced or eliminated by proper operating techniques.

The problem of lead fouling arises when low engine operating temperatures are coupled with running a rich mixture. Doing this prevents the complete vaporization of TEL. Under these conditions, lead deposits can form on spark plug electrodes, causing misfiring. By establishing and maintaining proper engine operating temperatures, the TEL can be kept properly vaporized and pass out the exhaust system.

For pilots experiencing lead fouling, the following operating recommendations are made:

  1. Use only the spark plugs that are recommended by the manufacturer.
  2. Rotate the top and bottom plugs every 25 to 50 hours.
  3. Proper adjustment of the idle speed (600 to 650 RPM) fuel mixture and maintenance of the air induction system will ensure smooth engine operations and eliminate excessive rich fuel/air mixture at idle speeds.
  4. The engine should be operated at speeds between 1000 to 1200 RPM after starting to reduce the initial warm-up period. Avoid long prolonged closed throttle idle engine speed operation (when possible).
  5. After a flooded start, slowly run the engine to high power to burn off harmful lead deposits, then return the engine to normal operating power.
  6. Keep engine operating temperatures in the normal operating range. Too often many people think the lower the temperature the better. Keep cylinder head temperatures in the normal operating range by use of normal power and proper leaning.
  7. Use normal recommended leaning techniques at cruise conditions regardless of altitude and re-lean the mixture with application of alternate air or carburetor heat. If aircraft are used as trainers, schedule cross country operations whenever possible.
  8. Rapid engine cool down from low power altitude changes, low power landing approaches and/or engine shutdown too soon after landing or ground runs should be avoided.
  9. Prior to engine shut-down the engine speed should be maintained between 1000 to 1200 RPM. until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000 to 1200 RPM and shut-down immediately using the mixture control.
  10. During ground and taxiing operations, lean the mixture.