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The presence of a rich condition in engine performance can be indicated by an increase in unburned hydrocarbons (HC) alongside a decline in carbon monoxide (CO) levels. A rich air-fuel mixture means there is more fuel than the ideal stoichiometric ratio, leading to incomplete combustion. In such scenarios, unburned fuel will show up as elevated HC emissions because there isn't sufficient oxygen to combust all the fuel.
When there is a large decrease in CO, it suggests that the fuel is being combusted more completely; however, if there is still a significant amount of HC, it reflects that not all fuel is being efficiently converted into CO2, typical of a rich mixture. This combination points towards a performance issue, likely related to an overly rich air-fuel mixture.
The other options do not accurately indicate a rich condition. Stabilized CO with increased CO2 would point to a well-tuned engine rather than a rich condition. A large increase in O2 could suggest a lean condition or a problem in the combustion process. Stable HC measurements imply that the combustion is consistent—neither overly rich nor lean. Thus, the context of emissions relative to combustion efficiency makes the relationship between CO, HC, and the air-fuel ratio