1. Idle Air Control (IAC) Valve Issues: The IAC valve regulates the amount of air that bypasses the throttle valve to maintain a steady idle speed. If the IAC valve is faulty or clogged, it can cause the engine to sputter or stall when the car comes to a stop. In park, since the car is not in gear, the IAC valve does not need to work as hard, and the sputtering issue might not be as noticeable.
2. Faulty Oxygen Sensor: The oxygen sensor monitors the amount of oxygen in the exhaust gases and provides feedback to the engine control unit (ECU) to adjust the air-fuel mixture. A faulty oxygen sensor can send incorrect signals to the ECU, causing the engine to run rich or lean, which can lead to sputtering when stopping at a light.
3. Vacuum Leaks: Vacuum leaks can cause the engine to run lean, leading to sputtering issues. When the car is in park and idling, the vacuum system may not be under as much stress as when the car is in gear and moving. This means that a small vacuum leak may not cause sputtering in park but may become more noticeable when you stop at a light.
4. Faulty Fuel Injector/s: A faulty fuel injector can cause irregular fuel delivery, resulting in a sputtering engine. When the car is in park, the engine might be able to compensate for the faulty injector, but the issue becomes more apparent when the car comes to a stop at a light.
5. Throttle Position Sensor (TPS) Issues: The TPS monitors the position of the throttle valve and communicates this information to the ECU. A faulty TPS can send incorrect signals, leading to improper air-fuel mixture and causing the engine to sputter when stopping at a light.
These are just a few of the possible reasons why a car may sputter when stopping at a light but idle fine in park. It is recommended to have the problem diagnosed by a qualified mechanic to identify the exact cause and implement the appropriate repair or maintenance.
