Raw fuel is primarily made up of hydrocarbons which should be completely broken apart during combustion, but imperfect combustion or engine issues can cause some hydrocarbons to pass through unburned out the tailpipe. The document lists several potential causes of incomplete combustion like engine integrity problems, ignition issues, fueling problems, excessive exhaust gas recirculation, and catalytic converter malfunctions.

1. • Raw fuel is primarily made up of Hydrocarbons (HC).
• Should be almost completely broken apart by the
combustion process.
• If there is a lack of combustion (misfire), some of the
Hydrocarbons pass through unburned and out the
tailpipe.
• Variations in the combustion event (imperfect
combustion) allow HC’s to pass through unburned.
• Catalytic Converter can only deal with a normal
amount of HC’s.

2. • Engine integrity issues (lack of compression, lack
of vacuum).
• Ignition timing.
• Ignition system malfunctions.
• Rich misfires (high fuel pressure, regulator and/or
injector leaks).
• Lean misfires (low fuel pressure, dirty
injector, vacuum leaks).
• Poor fuel vaporization.
• Air Injection system malfunction.
• Excessive EGR.
• Catalytic Converter operation