Regardless of the type of spark plug used by your car, chances are high that without proper engine maintenance, you could experience spark plug fouling. Fouled spark plugs cause misfires, rough idle, and a general degradation of performance in an engine. However, spark plugs can become fouled even with proper maintenance due to unforeseen engine problems.
In addition to normal wear, fouling can lead to the signs it's time to replace your spark plugs. These signs can take many forms, each with different causes, ranging from ash deposits to oil fouling and carbon buildup. Champion Auto Parts says oil fouling is the result of oil entering the combustion chamber.
"Oil deposits cover the spark plug which can keep the spark from arcing across the gap. Instead it takes the shorter path to ground through the oil. Usually a sign of advanced engine wear."
Fuel fouling causes similar issues to oil fouling and typically stems from a fuel/air mixture that is rich with fuel. Dirty injectors, clogged air filters, or bad sensors can cause the fuel/air mixture to be off and lead to fuel fouling.
Carbon fouling is another issue that can result from a rich fuel/air mixture. It's sometimes caused by a weak ignition spark, using a spark plug with a heat range that is too cold, low-quality gasoline, or taking frequent short drives. The common theme among these types of fouling is that they all interfere with the spark required to ignite the combustion chamber's fuel/air mixture. This leaves fuel either completely or partially unburned.
If you're just trying to do some yardwork or get a dirtbike back to the trailhead, cleaning and reusing old spark plugs can suffice in a pinch. However, replacing fouled plugs is a better long-term solution. Good steps towards preventing spark plug fouling altogether include keeping up with your vehicle's maintenance and replacing faulty sensors.
Shop Owners Mag says:
"If the mass air flow sensor or oxygen sensors are not accurately reporting the air that's coming into the engine or the oxygen content in the exhaust stream, it could cause a rich-running condition that can cause carbon to foul the spark plugs."
Carbon, in the form of carbon dioxide and carbon monoxide, is naturally produced from burning gasoline during the combustion process. However, in some cases, the excess carbon in unburned gasoline forms sticky deposits within the combustion chamber, on spark plugs, and on the intake valves of direct injection engines, leading to fouling.
Preventive measures include using high quality gasoline and reducing the frequency of short drives as carbon deposits can be reduced as the engine heats up on longer drives. Fuel additives may also help prevent or break up existing carbon deposits.