What is a primary cause of lack of Oxygen (O2) during engine combustion?

A rich air-fuel mixture is indeed a primary cause of a lack of oxygen during engine combustion. When the air-fuel mixture contains an excess of fuel compared to the amount of air, it results in insufficient oxygen being available for the combustion process. This imbalance can lead to incomplete combustion, causing the engine to run inefficiently and producing higher emissions of unburned hydrocarbons and carbon monoxide.

In an ideal combustion process, the correct ratio of air to fuel allows for the complete burning of the fuel, maximizing engine performance and reducing harmful emissions. However, when the mixture is too rich, it indicates that there is not enough air present for the amount of fuel, consequently limiting the combustion efficiency and oxygen availability.

Other factors might contribute to combustion efficiency, but they do not directly result in a lack of oxygen in the same way that a rich mixture does. For instance, vacuum leaks can introduce extra air into the engine but do not necessarily lead to a lack of oxygen; rather, they can disrupt the air-fuel mixture. Similarly, fouled spark plugs can hinder ignition but are not a direct cause of oxygen shortage. High engine temperatures primarily affect performance and can lead to excessive wear or damage, but they don't directly limit the available oxygen for combustion