Which condition leads to an abundance of Oxygen (O2) within an engine during combustion?

The condition that leads to an abundance of Oxygen (O2) within an engine during combustion is a vacuum leak. A vacuum leak refers to an unintended opening in the intake manifold that allows excess air to enter the engine without being measured by the air intake system. This leads to a dilution of the air-fuel mixture, which can cause more oxygen to be present than what the engine can effectively utilize for combustion.

When a vacuum leak occurs, the engine management system receives signals indicating that less air is entering the engine than is actually the case. As a result, the system continues to provide fuel based on the incorrect airflow readings, leading to a lean air-fuel mixture characterized by an increased amount of unburned oxygen in the exhaust. This is opposed to a rich air-fuel mixture, where there is an excess of fuel relative to the amount of oxygen, resulting in less unburned oxygen available.

Thus, the presence of a vacuum leak creates a condition in which the combustion process does not optimally utilize the available oxygen, leaving excess oxygen in the exhaust gases due to incomplete fuel combustion and an uneven fuel distribution within the combustion chamber.