Question about Subaru Impreza WRX STi
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Posted on Jan 02, 2017
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Ensure all connections are clean and properly tightened.
Mass air flow sensor
A mass air flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the Failure Records conditions with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the Mass Airflow Sensor Replacement .
Damaged accessory drive belt or driven accessory
A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
Air induction system
Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, exhaust gas recirculation (EGR) valve, and intake manifold mounting surfaces.
Perform a fuel system pressure test. A malfunctioning fuel pump, a plugged filter, or a malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis .
Refer to Fuel Injector Solenoid Coil Test .
Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire, refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80?°C (176?°F), the condition described above is very likely the cause of the DTC to set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s.
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