It is possible to diagnose a variety of engine conditions by examining the electrodes of the spark plugs. Ideally, all of the plugs from an engine should look alike. Whenever plugs from different cylinders look different, a problem exists in those cylinders.
Some engines are equipped with spark plugs that have more than one ground electrode. The gap between the center electrode and each ground electrode should be checked.
The gap between the center electrode and both ground electrodes should be checked and adjusted to specifications. Reprinted with permission.
If the gap between the center electrode and one of the ground electrodes is less than the others, spark will occur only at the smallest gap.
Always check the air gap of a new spark plug before installing it. Never assume the gap is correct just because the plug is new.
Once the spark plugs have been removed, it is important to "read" them.
Carefully inspect each spark plug.
In other words, inspect them closely, noting in particular any deposits on the plugs and the degree of electrode erosion. A normal-firing spark plug will have a minimum amount of deposits on it and will be colored light tan or gray.
Normal spark plug. Courtesy of Champion Spark Plug Company.
However, there should be no evidence of electrode burning, and the increase of the air gap should be no more than 0.001 inch (0.0254 mm) for every 10,000 miles (16,000 km) of engine operation. A plug that exceeds this wear should be replaced and the cause of excessive wear corrected.
A worn spark plug. Courtesy of Champion Spark Plug Company.
Worn or dirty spark plugs may work fine at idle or low speeds, but they frequently fail during heavy loads or higher engine speeds.