Final answer:
An engine could run lean with an adequate fuel supply due to issues like low octane rating, vacuum leaks, or environmental temperature effects leading to increased air intake or decreased fuel density. Issues with diesel engine compression can also cause lean operation.
Step-by-step explanation:
An engine would run lean even with a normal amount of fuel present if there's an improper balance in the air-to-fuel ratio, particularly if there is too much air in the mix. This can occur for several reasons, such as vacuum leaks, incorrect settings on fuel injection systems, or issues within the engine that allow additional air to enter the combustion chamber outside of the normal intake process.
Other factors could also contribute to a lean-running engine. For instance, if gasoline has an octane rating that is too low for an engine, this can result in premature combustion, where the fuel burns unevenly, causing knocking and pinging noises. The improper octane level affects the combustion process even if the amount of fuel is adequate.
Environmental conditions such as temperature can also impact how fuel is utilized in an engine. During the summer, the thermal expansion of gasoline may reduce its mass even though its volume remains the same. Consequently, the engine may run leaner as there is less mass of fuel being injected into the combustion chamber compared to when temperatures are lower.
Moreover, in the case of a diesel engine, where fuel is ignited by means of compression rather than a spark, variations in the intake air temperature and the resultant compression temperature can influence combustion. If the conditions are such that the air-fuel mix isn't optimal, it can lead to a lean mixture even when diesel fuel is adequately supplied.
In any of these situations, although the volume of fuel may seem consistent, various factors are affecting the actual fuel-to-air ratio, leading the engine to operate lean. Addressing these issues typically requires mechanic intervention to correct the underlying causes, such as replacing or repairing malfunctioning parts, adjusting settings, or using fuel with the correct octane rating.