Spark Plug Problems

The following is derived from maintenance facility submissions to the FAA’s Service Difficulty Reports database.

Editorial Staff

Saturday, June 08, 2024 at 08:00 AM ET

Verified

Key Takeaways:

Multiple aircraft experienced significant engine issues, including misfires, preignition, and power loss, directly linked to various spark plug failures.
Common spark plug defects cited include internal resistor failures (leading to excessive or zero resistance), insulator failures, and the physical departure of a plug from its cylinder.
These spark plug malfunctions are also implicated in subsequent component failures, such as repeated magneto failures, sometimes occurring at relatively low operational hours.

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Cessna 172N Skyhawk

Internal Resistor Failure

The pilot noticed a high EGT reading. A magneto check revealed one of the eight spark plugs was not firing. Examination revealed resistance in five of the eight plugs exceeded the 2 MΩ limit. The internal resistors had failed and the current path was open. The resistors should have been about 1.5 kΩ.

An open current path causes extremely high voltages in the ignition system that can lead to in-flight engine failures. The short service life of magnetos experienced in recent years may have been caused by defective plugs.

Part total time: 300.0 hours

Cessna 402C Utililiner

Suspected Insulator Failure

The pilot reported the left engine would only develop 26 inches of manifold pressure and vibrated. Inspection of the left engine revealed a preignition event in the #3 cylinder, which caused significant damage to the piston, requiring cylinder replacement.

Suspect spark plug insulator failure to be the cause of this event.

Part total time: Unknown

Mooney M20U Ovation

Magneto Failures

A string of three in-flight magneto failures over approximately 100 flight hours was linked to bad Tempest fine wire spark plugs, one of which lost a center electrode in flight during engine operation. Since replacing the plugs, approximately 100 flight hours have elapsed with no more magneto failures or abnormal operation.

Part total time: 500.0 hours

Piper PA-25 Pawnee

Departing Spark Plug

During cruise, the #3 cylinder’s lower spark plug (p/n SLSTD1872), departed the engine, causing a substantial loss of power. The suspect spark plug was recovered, and was found to have the helicoil from the cylinder still firmly attached.

Part total time: 51.0 hours

Piper PA-46-350P Malibu Mirage

Stumbling Engine

The pilot reported the aircraft seemed to be short on power during climb. An in-flight magneto check revealed rough operation on the left magneto. Inspection of the magnetos and harness was satisfactory. Spark plugs were removed, inspected and tested with a standard spark plug machine, with no discrepancies noted. Each spark plug was then tested using an ohmmeter; several had excessive resistance, with a couple having no resistance. There is no published value for this parameter, but various suppliers and groups confirm resistance affects plug operation.

Part total time: 895.0 hours