Anatomy of a spark plug
A spark plug basically consists of 5 components:
How a spark plug works
The engine ignition control circuit determines when the spark is to occur. At this time, a very high voltage pulse is delivered to the spark plug terminal end. The voltage is so high that electrical current can flow across the gap between the center electrode and the ground electrode, resulting in a spark. Of course, it is this spark which ignites the fuel-air mixture that has been drawn into and compressed in the engine combustion chamber. However simple a spark plug may be, the construction is critical. A spark plug must be able to withstand frequent high voltage pulses, high pressures, high temperatures, vibration and combustion by-products. Each of these variables contribute to the possible failure of a spark plug.
How a spark plug fails
A spark plug can fail in three basic ways. 1) mechanical failure, 2) fouling, or 3) overheating.
Mechanincal failure of a plug would likely be a broken insulator, stripped threads, damaged sealing surface or sealing ring, or a broken terminal end. These failures are likely to occur from poor handling or careless installation and are usually obvious to visual inspection. While possible, mechanical failures are not likely occur when installed correctly.
A spark plug can fail by what is referred to as fouling. Spark plug fouling occurs when the insulator nose at the center electrode accumulates either fuel, oil or carbon. This accumulation allows the high voltage to short circuit along the insulator nose to ground rather than jumping the electrode gap. A fouled spark plug can sometimes be cleaned to restore normal operation. Often, however, replacement is the only solution.
Possible causes of fouling:
A spark plug can fail by overheating, which can cause the electrodes to wear quickly, melt or break apart. The failure mode of the electrode depends in part on the type of electrode material. Engine overheating has an array of causes and harmful effects not limited to a spark plug.
Possible causes of over heating:
Visual examples of failed spark plugs, courtesy of NGK:
How to test spark plugs
It is possible for a spark plug to fail yet appear normal. Using an ohm meter, you can check a suspected bad plug.
1 Connect one test lead to the spark plug terminal and the other lead to the center electrode (note: you may need to clean the center electrode to have good continuity with the meter lead). If you have a resistor type spark plug, you should have a reading of several thousand ohms (around 5k ohms, plus or minus 1k ohm or so). If you have a non-resistor type spark plug, the meter should display very low ohms or zero. If this measurement shows an open circuit condition, the plug should be replaced. If the measurement indicates a good spark plug, proceed to the next test.
Click below for graphic view:
2 Leave the test lead on the spark plug terminal end and move the other lead the ground electrode or any place on the metallic body, below the ceramic insulator. It is important that you keep at least one hand from touching the plug or test lead tip to avoid a false reading. This measurement should show an open circuit measurement. Any measured resistance (even in the meg ohm range) is bad and will cause the high voltage pulse to bypass the high resistance air gap, resulting in no spark or weak spark.
Click below for graphic view:
Please consult manufacturer technical specifications for any given spark plug.
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