What is meant by "running rich" in terms of an engine?

"Running rich" refers to a condition in an engine where there is too much fuel in the air-fuel mixture. In a properly functioning internal combustion engine, a precise balance between air and fuel is crucial for optimal performance, combustion efficiency, and emissions control. When an engine runs rich, it typically means that the fuel-to-air ratio exceeds the ideal proportion necessary for complete combustion.

This can manifest in several ways, including a noticeable decrease in fuel economy, an increase in emissions (such as hydrocarbons and carbon monoxide), and potentially lead to engine issues over time, such as fouled spark plugs or damage to the catalytic converter. Proper tuning and adjustment of the fuel delivery system or air intake might be necessary to correct a rich condition. Understanding this concept is essential for emissions inspectors, as identifying a rich-running engine can indicate potential compliance issues with environmental regulations.