The spark plug has a tough job to do, and it has to do it under extreme conditions. The terminal (top) end of the spark plug is connected to a high voltage supply source, the ignition coil. As current flows from the coil, a voltage potential builds between the center (positive) electrode and the side (ground) electrode.
Now, of course, the two electrodes do not touch and that means it’s an open circuit, right?
Not entirely. There is something there—air. Or more precisely, at the moment of ignition, the air/fuel mixture that was injected into the cylinder micro-moments before is present. Initially, this mixture acts as an insulator but as the voltage potential grows, the gases begin to change. When the dielectric strength of the gases is reached, the gases ionize and become a conductor instead.
No more open circuit!
As the current flows across the now-bridged gap, the temperature in the spark channel rises significantly and quickly. This causes the ionized gases to expand just as quickly, like a small explosion, and results in the “click” you hear when a spark jumps outside of the combustion chamber. It’s a lot like lightning and the resulting thunder you hear during a summer storm.
The heat and pressure force the gases to react with one another and ideally, near the end of the spark event, a small ball of fire (or kernel) has formed as the gases burn on their own. This tiny little fireball then expands to consume the surrounding gases and combustion is realized.
The spark plug plays a critical role in the combustion process. Even minor discrepancies in the ability of the plug to function can result in inefficiencies of the process—and those lead to hard-to-nail-down, intermittent misfires.
For example, consider a gap that is too big or too small for the application. If it’s too small, the resulting spark may be too weak to ignite the air/fuel mixture. The spark will fire each ignition cycle but still be the root cause of that elusive miss. If it’s too large, the voltage requirement goes up, perhaps to the point that the plug doesn’t “fire” at all. Excessive gaps, either due to improper initial setting or wear, can also stress ignition coils to the point of failure.
When servicing spark plugs, begin by removing the coil or spark plug lead and inspect the plug boot for any visual signs of damage or arc-through and repair as needed. Use compressed air to clean the area around the base of the plug of any accumulated debris prior to removing the plug. We don’t want anything to drop down into the cylinder, do we?
Once removed, inspect the plug for signs of damage—and to make sure the entire plug came out!
While not as common (nor critical) as it once was, it’s still not a bad idea to “read” the plug for clues to any drivability issues you may be troubleshooting.