Technician A says poor idle drivability can result from mechanical failures, electrical opens or shorts, or improper tuning calibrations. Technician B says that during idle the PCM commands spark advance for low-end torque. Which technician is correct?

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

Answer: (D)

Idle stability depends on a precise balance of air, fuel, and ignition, all maintained by the engine’s mechanical condition, electrical sensing, and the calibration in the PCM. Mechanical failures (like worn components, leaks, or timing chain/valve issues) can upset the air-fuel mixture and timing, while electrical opens or shorts can cause sensors to feed incorrect data or fuel to be miscontrolled—both leading to rough idle or stalling. Improper tuning calibrations can throw off fuel tables, idle target speeds, or spark timing, producing similar idle problems. In addition, the PCM typically uses some spark timing at idle to ensure smooth operation and adequate low-end torque; this timing strategy is deliberate and helps prevent stalling, but it must be within the correct range. Taken together, these factors show that both technicians are describing valid aspects of idle drivability.