We all know that an automotive spark plug is a device that forces a high-voltage charge of electricity to arc across a gap, thus igniting the air/fuel mixture in the combustion chamber. What may come as a surprise is twofold. First, simply inspecting these little devices can be used to help diagnose engine issues. Seriously. Second, spark-plug technology has grown exponentially over the past several years to a point where simply installing a high-tech set of plugs can make a positive difference in your rig's power and fuel economy. Let's take a look.
Part 1: Engine DiagnosisJust as Jimmy Nylund found that analyzing engine oil can uncover important internal engine information ("Oil Analysis," Sept. '06), inspecting your spark plugs can reveal a lot about what's going on inside your engine. After the plugs have been used in an engine for several thousand miles, they harbor telltale data. The plug face (electrode end) can offer insight into the overall condition of items such as piston rings and valve seals. They can also help identify issues with things like the carburetor, injectors, valves, or vacuum advance. The plugs can allow you to diagnose a variety of fouling scenarios as well as pre-ignition, overheating, low operating temperature, gap bridging, plugs with the wrong heat range, or even some general mechanical damage. Here are some commonly found problems and causes:
Ash-fouled: Loose to cinder-like heavy ash deposits on the insulator nose. Alloying constituents, particularly from engine oil, can deposit this ash in the combustion chamber and on the plug face. This can lead to auto-ignition, with loss of power and possible engine damage. If the engine isn't damaged, you can change the oil and fit new plugs.
Problem: None
Here you can see a normal used spark plug. Note how the combustion deposits are slight, and they won't have any effect on engine performance. The plug has a brown to grayish-tan color, indicating that the plug is in the correct heat range and operating in a healthy engine. | 
Problem: Bridging
This rarely occurs in engines used on-road. During aggressive off-highway driving, combustion deposits can be literally thrown loose and then they may lodge between the electrodes. Another cause of bridging is when fluffy materials that accumulate on the side electrode melt and bridge the gap when the engine is suddenly put under heavy load. Both conditions may result in a dead short or misfire. | 
Problem: Excessive wear
This plug is worn out. Note how both electrodes are showing extreme wear, thereby creating a huge gap. Naturally, this large gap will affect how the plug performs. The voltage required to fire a plug like this has effectively doubled and will continue to increase the longer the engine operates. Further, even higher voltage requirements may occur when the engine is accelerated quickly. Expect poor engine performance and a loss of fuel mileage. |