What is the most likely cause of having a normal oxygen sensor reading at idle but a rich reading during acceleration?

A normal oxygen sensor reading at idle coupled with a rich reading during acceleration points to an inconsistency in the fuel delivery system. The most likely cause of this scenario is a fuel injection malfunction. During idle, the engine operates under controlled conditions and the fuel injection system is able to compensate effectively, allowing the oxygen sensor to read a normal level. However, when accelerating, if there is an issue with the fuel injectors—such as them delivering too much fuel or improperly atomizing the fuel—this can create a richer air-fuel mixture than optimal. Consequently, the oxygen sensor detects elevated levels of unburned fuel, resulting in a rich condition.

In contrast, correct engine tuning is unlikely to cause this condition since proper tuning helps maintain an appropriate air-fuel mixture across different operating conditions. A normal combustion process would not yield rich readings during acceleration unless something is disrupting the mixture balance. Lastly, an exhaust system blockage typically would affect both idle and acceleration conditions – not just create an issue during acceleration. This reinforces that a malfunction specifically related to the fuel injection is the most plausible cause of the observed symptoms.