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2003 Pontiac Montana AWD 3.4L - P0300, P0302/P0303 & P0440 Code: Misfire and Evaporative Emissions Issue at 104k Miles

Model: 2003 Pontiac Montana AWD Fault Code: P0300 Posted: 2010-12-13 09:34

2003 Pontiac Montana AWD with a 3.4L engine, currently at 104,000 miles. Recently experienced consistent P0300 (Random Misfire), along with intermittent P0302 and P0303 codes. These codes are cleared and reappear shortly after driving, indicating recurring misfiring. Additionally, a P0440 code has been detected — labeled as Evaporative Emissions System Malfunction, though the description in the diagnostic trouble code list suggests an issue with the evaporative system. The engine runs but exhibits noticeable misfire symptoms and unusually high fuel consumption. The gas cap and O-ring appear to be in good condition. Previously, I experienced rare P0300 occurrences during heavy acceleration. A plug #5 blew out approximately four months ago and was repaired using a helix coil kit; since then, new Bosch Iridium plugs and MSD performance wires were installed with proper gap settings. I initially suspected ignition system issues such as distributor or coil pack failure due to the random misfire pattern. However, the presence of P0440 has led me to consider whether evaporative emissions components could be contributing to cylinder misfires — though this is uncommon unless purge flow enters the intake manifold. I have noticed a faint exhaust smell but no direct fuel odor. Any insights or troubleshooting suggestions would be greatly appreciated.

Related fault codes
P0300P0302P0303
Comments (2)
Anonymous 2010-12-13 17:59

Evaporative emissions system issues typically do not cause engine misfires unless there is unintended purge flow into the intake manifold, which is relatively rare. To test this scenario, you can temporarily block the vacuum hoses connecting the intake to the evaporative purge solenoid and monitor whether drivability or misfire symptoms improve. Could you please specify the brands of your 'high-performance' plugs and wires? For example, are they Bosch Iridium or MSD? Also, were the spark plugs properly gapped? You mentioned a blown plug was repaired with a helix kit — could you clarify how that repair was performed? Was it a helical coil replacement? Additionally, would you recommend testing the coil pack and ignition module to rule out electrical faults?

Anonymous 2010-12-14 09:23

Thank you for your response. I should have clarified that 'high-quality' rather than 'performance' was intended — the plugs are Bosch Iridium, and the wires are MSD. All spark plugs were properly gapped according to manufacturer specifications. The blown plug #5 was repaired using a helix coil kit, which resolved the issue at the time and allowed for smooth operation afterward. Given that the misfire is random and not specific to one cylinder, I suspect an ignition system fault — possibly related to the distributor or coil pack. I’ve already double-checked all spark plugs and confirmed that wires are securely connected and properly installed.