Which technician correctly described which factor can cause oscillations in the data log during idle?

• A. Tech A
• B. Tech B
• C. Both
• D. Neither

Answer: (A)

Oscillations in the idle portion of the data log show up when the engine-control loop that keeps idle steady becomes unstable because the input it relies on is continually changing. The idle loop continuously adjusts air and fuel to hit a target RPM, and if that input signal wobbles—such as from an intermittent vacuum leak or noisy sensor data—the ECU over- and under-corrects. Those repeated adjustments show up as rhythmic swings in the log during idle.

Tech A’s description aligns with this because they pointed to a factor that introduces fluctuating input to the idle control loop. That kind of instability is exactly what drives the oscillations you see. Factors that don’t introduce a variable input to the idle loop, or that only affect non-idle conditions, wouldn’t produce the same oscillatory pattern in the idle section of the log.