If hydrocarbon (HC) levels are acceptable at idle but not under load, what could be a potential cause?

When hydrocarbon (HC) levels are acceptable at idle but increase under load, it suggests that while the engine runs efficiently at low RPMs, it struggles when more power is required. This situation can be indicative of multiple potential issues affecting fuel combustion, particularly during higher engine demands.

Excessive plug wire resistance is a factor that can hinder the performance of the ignition system. Under load, the increased demand for spark energy may expose weaknesses in the spark plug wires, leading to a weaker spark. A weak or inconsistent spark could result in incomplete combustion, which in turn would increase HC emissions.

Fouling of the spark plug can also lead to higher HC emissions under load. If a spark plug is fouled, it can misfire or fail to ignite the air-fuel mixture properly, particularly when the engine is under load. This scenario can result in unburned hydrocarbons being expelled in the exhaust.

A rich condition, where there is too much fuel relative to air in the combustion chamber, can result in excessive hydrocarbons especially under load conditions where the engine demands more power. This can cause incomplete combustion and higher HC emissions as well.

Thus, the relationship between excessive plug wire resistance, fouled spark plugs, and a rich condition can