Diagnosing Spark Plugs

The spark plug affects entire engine performance and therefore its inspection is very important.
- Check electrode and insulator for presence of cracks, and replace if any.
- Check electrode for wear, and replace if necessary.
- Check gasket for damage, and replace if necessary.
- Measure insulation resistance with an ohmmeter, and replace if faulty.
- Adjust spark plug gap to 1.0 mm (0.04 in) 1.1 mm (0.043 in).
- Check fuel and electrical systems if spark plug is extremely dirty.
- Use spark plugs having low heat value (hot type plug) if fuel and electrical systems are normal.
- Use spark plugs having high heat value (cold type plug) if insulator and electrode are extremely burned.

Sooty Spark Plugs
Much deposit of carbon or oil on the electrode and insulator of spark plug reduces the engine performance.

Possible causes:
- Too rich mixture
- Presence of oil in combustion chamber
- Incorrectly adjusted spark plug gap

Burning Electrodes
This fault is characterized by scorched or heavily oxidized electrode or blistered insulator nose.

Possible causes:
- Too lean mixture
- Improper heat value

Measuring Insulation Resistance
- Measure insulation resistance using a 500 volt megaohm meter.
- Replace spark plugs if measured value is out of standard.

Insulation resistance: 50 M ohms or more

Cleaning Spark Plugs
- Clean spark plugs with a spark plug cleaner.
- Raise the ground electrode to an angle of 45 to 60 degrees. If electrode is wet, dry it before cleaning.
- After spark plug is thoroughly cleaned, check insulator for presence of cracks.
- Clean threads and metal body with a wire brush.
- File the electrode tip if electrode is extremely worn.
- Bend the ground electrode to adjust the spark plug gap.