Spark Plug Operation Failures

3-03 Analyze spark plug service failures.

If spark plug operation is degraded for any reason, the degradation reduces the engine’s ability to produce maximum engine power and increases emissions output due to incomplete combustion. Spark plugs wear, much like brake pads or a clutch; they are made to wear. Over the lifetime of the spark plug, a little bit of the metal is worn away with each arch, which over time will cause the gap or the materials in the spark plug to become too degraded to perform at the level it or they should. If there is a failure within the cylinder, it increases the wear on the spark plug and will thus need to be replaced sooner than if the cylinder had been operating at optimum efficiency all the time.

Poor maintenance/deposits: Spark plugs that remain in the cylinder past the recommended service intervals can cause problems. Deposits accumulate on the spark plug’s firing end during regular use, creating hot spots (FIGURE 3-5). The hot spots induce abnormal combustion (pre-ignition) and can cause the electrodes to melt. Deposits also collect on the insulator body. The deposits form a low resistance path to ground. The spark follows the deposits along the insulator to the cylinder head instead of jumping the plug gap and igniting the air-fuel mixture, creating a misfire. This is called carbon tracking.

FIGURE 3-5 This condition can be caused by overtemperature inside the combustion chamber or an ignition timing problem.

Worn electrodes: During normal operation, every time a spark plug fires, the spark erodes a microscopic amount of metal off the plug’s electrodes. Over time, the constant firing and erosion of the metal rounds the spark plug’s sharp edges, increasing the distance between the electrodes. As the gap widens, the amount of voltage required to fire the spark plug increases (FIGURE 3-6). Ultimately, the amount of voltage needed to jump the gap exceeds the voltage output of the coil, and then the spark plug misfires. Typically starting as a misfire under load, the condition gradually becomes worse, and the misfire may be present at all times. Worn spark plugs are also prone to causing a no-start or hard-starting condition. Starting problems increase during cold ambient temperatures. Cold temperatures increase the voltage required to jump the spark plug gap, further stressing the coil.

FIGURE 3-6 This type of wear is normal, and the spark plug should be replaced because it is out of specification.

Fouled: Spark plugs gradually lose their ability to burn carbon deposits due to normal accumulation as they wear. Carbon on the firing end of the spark plug is a conductor. Spark plug fouling occurs when carbon that attaches to the firing end of the spark plug creates an easier path to ground than the harsh environment that is present during the compression stroke. The high voltage produced by the coil “leaks” its way to ground through the carbon, bypassing the spark plug and thus creating a misfire. The severity of the misfire is directly related to the amount of fouling present on the spark plug (FIGURE 3-7).

FIGURE 3-7 As the spark plug becomes fouled as it is used, it should be replaced as a maintenance item like everything else on the engine.

Carbon tracking: A common fault with modern COP ignition systems is carbon tracking or flashover on the spark plug insulator. Carbon tracking causes a misfire/stumble that is usually most noticeable during light acceleration. A misfire caused by carbon tracking may disappear as the load increases.

Carbon tracking has multiple causes: an excessive spark plug gap from erosion, oil, dirt, and moisture that combines with air that ionizes in the boot. Severely worn spark plug electrodes increase the firing voltages to the point that electrical energy looks for another, easier path to ground. The path taken can be either down the insulator to the metal shell and then the cylinder head or through the coil/plug boot. With use and age, the coil or spark plug boot hardens, decreasing its sealing ability and thus allowing contaminants in, which increases the likelihood of flashover. The reduced sealing ability lowers the resistance to carbon tracking. To promote sealing, apply dielectric grease to the plug boot and plug tip.

When carbon tracking is found on the spark plug, an identical trace will be on the coil boot or spark plug wire boot (FIGURE 3-8). Failure to replace the boot or wire along with the spark plug will transfer the carbon to the new plug. The carbon track will worsen over time, repeating the misfire. Coil boots that can be replaced separately should be replaced at regular intervals due to the extremely harsh environment that they operate in. Routine maintenance will help reduce carbon tracking.

FIGURE 3-8 Inspecting the spark plug for carbon tracking may indicate a crack in the insulator or a spark plug boot that is allowing contaminants between it and the insulator.

TECHNICIAN TIP

Most vehicle and spark plug manufacturers do not recommend using antiseize (or any lubricant) on the spark plug threads during installation. Antiseize acts as a lubricant, reducing the effort required to torque the spark plug to specification. A reduction in torque of 20–40% increases the risk of over-tightening and thus breaking the spark plug.

TABLE 3-1 Reading Spark Plugs: A Diagnostic Chart

TABLE 3-2 Spark Plug Installation and Fault Removal