Pre-ignition is a condition whereby combustion is initiated within the combustion chamber before the spark occurs at the plug. This results in uncontrolled ignition and combustion conditions.

If a spark plug operates at a temperature in excess of 1000 degrees Celsius for a prolonged period the electrodes can start to overheat. When excessively overheated, pre-ignition can occur, the electrodes and insulator tip may melt and piston damage may result.

Pre-ignition is usually caused by a temperature exceeding
850 degrees Celsius – 1000 degrees Celsius (1600 degrees Fahrenheit – 1800 degrees Fahrenheit)

Causes of overheating

• Over-advanced ignition timing
• Too lean fuel mixture
• Excessive deposits accumulated in combustion chamber
• Insufficient cooling
• Insufficient spark plug tightening or failure to fit gasket
• Too low octane gasoline
• Too hot a spark plug fitted

Overheated

The plug has been subjected to a relatively heavy load and evidence of overheating can be seen in the oxidized electrodes and melted deposits which have formed on the insulator surface.

Recommendation
Check for over-advanced ignition timing and too lean fuel mixture. Check spark plug tightening and gasket. If conditions recur, use plug one step colder in heat range.

Melted

Overheated plug with melted electrodes and blistered ceramic insulator surface.

Recommendation
Check for over-advanced ignition timing and too lean fuel mixture. Check spark plug tightening and gasket. If conditions recur, use plug one step colder in heat range.

Worn Spark Plug

A worn spark plug not only wastes fuel but also loads the whole ignition system because the expanded gap requires higher voltage.

As a result, a worn spark plug may also cause engine damage and increased exhaust emissions.

Recommendation
Spark plugs should be replaced.

Deposits

The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and engine operating period.

Recommendation
Check for excessive amounts of lubricating oil entering into the combustion chamber. High quality oil should be used.

Spark plug tip temperature