Skip to main content
TechConnect logo Service Bulletins

Spark Plug In Cylinder Flash Over

Bulletin 1011

Download PDF


All engine applications that use spark plugs


This bulletin will explain and visually show one failure mode of spark plugs adding confidence when replacement is needed


Spark plugs can fail and will need replacement at their recommended mileage interval due to wear. But excessive idle time and constant short trips will shorten the spark plug life cycle due to fuel deposits or fouling due to oil, coolant, or chemicals. The shortened life cycle may also be caused by the spark plug not reaching self-cleaning temperature on a regular basis. The ceramic that surrounds the center electrode starts to accumulate deposits providing the voltage an easier path to ground causing internal flashover and misfires.


If internal flashover should occur, regardless of the sharpness of the electrodes, the spark plug will need to be replaced. The flashover will cause low power, poor fuel economy, increased tail pipe emissions and eventually misfires. Internal flashover usually will not require spark plug wire or coil boot replacement but be sure to check the condition of these parts before reassembly.


Below are examples of internal flashover with figure 1 showing the voltage traveling down the internal ceramic insulator to the spark plug shell. Figure 2 and 3 illustrate the result of the voltage traveling across the ceramic and leaving trace marks in the ceramic as the voltage finds the easiest path to ground which is the spark plug shell. These spark plugs will require replacement.


For addition support contact the GoTech technical services team at 855-207-5630 M-F 7:30-6:00 CST for assistance.

Some content requires Adobe Acrobat Reader to view.