This enriches the fuel mixture and will foul the spark plugs. Most charcoal canisters have a filter that should be replaced periodically. This system should be checked when fuel mileage drops. This increases hydrocarbon emissions. To eliminate this source of emissions an air injection system was created.
Combustion requires fuel, oxygen and heat.
Without any one of the three combustion cannot occur. Inside the exhaust manifold there is sufficient heat to support combustion, if we introduce some oxygen than any unburned fuel will ignite. This combustion will not produce any power, but it will reduce excessive hydrocarbon emissions. Unlike in the combustion chamber, this combustion is uncontrolled, so if the fuel content of the exhaust is excessive, explosions, that sound like popping, will occur. There are times when under normal conditions, such as deceleration, when the fuel content is excessive.