Owner's Problem Description
The owner reports that their 2003 Chevy Impala experiences consistent stalling after about ten minutes of driving. Restarting is difficult; it only restarts after sitting for approximately five minutes. The engine idles roughly between 500-1100 RPM, often feeling on the verge of dying. A scan code indicates a misfire in cylinder 5, which seems intermittent. Previous repairs include replacing spark plugs, wires, swapping coils for testing, a new fuel filter, cleaning injectors and combustion chambers, and installing a new catalytic converter.
Professional Technical Analysis
The 2003 Chevy Impala, equipped with a 3.4L V6 engine and approximately 120,000 miles, presents an intermittent stalling issue after warming up, with rough idle symptoms. The vehicle exhibits fluctuating idle speeds from 500 to 1100 RPM, indicating potential fueling or ignition issues. The misfire detected in cylinder 5 suggests a localized problem, possibly exacerbated by prior repairs including spark plug and wire replacements, as well as a new catalytic converter installation. Notably, the engine's compression on cylinder 5 was reportedly higher than on cylinder 3 during previous tests, indicating potential underlying issues such as valve sealing or head gasket integrity. Engine performance may also be impacted by the cleaning of the combustion chamber, which could temporarily mask symptoms rather than resolve the root cause. The intermittent nature of the misfire, combined with rough idling behavior, typically points to fuel delivery issues, faulty ignition components, or even an electrical problem affecting the PCM (Powertrain Control Module). System interconnections must be considered, particularly how the fuel injectors and ignition coils interact with engine management systems. The vehicle’s current condition raises safety concerns due to stalling during operation, especially in traffic situations, indicating an urgent need for thorough diagnostics and repairs. Historical repair records indicate significant prior maintenance, suggesting that while the vehicle has been serviced, the root cause of the misfire and stalling remains unresolved.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Coil Issue: The ignition system may be experiencing failure in the coil associated with cylinder 5. This is common in aging vehicles, where heat can cause internal breakdowns. Testing should include resistance checks of the ignition coil (should be between 0.5-1.5 ohms for primary side) and secondary output voltage during operation, ideally above 10kV. - Check engine light activation, misfire code specific to cylinder 5, intermittent rough idling.
- Fuel Injector Failure: The fuel injector for cylinder 5 may be clogged or malfunctioning, leading to improper fuel delivery. Performing a flow test can help determine if the injector is operating within specifications (ideally 30-50 lbs/hr). If the resistance is out of spec (12-16 ohms), replacement may be necessary. - Rough idle, specific misfire codes, and intermittent performance issues after cleaning.
- Engine Mechanical Issue: A possible mechanical issue such as a burnt valve or head gasket problem could be causing compression inconsistencies leading to performance issues. Compression testing should reveal readings (ideally above 150 psi) across all cylinders to ensure uniform pressure. - Compression test results indicating higher values in cylinder 5 compared to adjacent cylinders.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - OBD-II System Scan: Start with a complete OBD-II scan using a professional diagnostic scanner or the GeekOBD APP to retrieve all diagnostic trouble codes (DTCs). Pay attention to any freeze frame data and monitor live data for real-time parameters, especially misfire counters and fuel trim values.
- STEP 2 - Ignition System Testing: Test the ignition coil associated with cylinder 5 for proper resistance (0.5-1.5 ohms for primary) and secondary output voltage during operation. Swap coils if necessary to isolate the issue. Document voltage readings and observe for changes in idle quality.
- STEP 3 - Fuel Injector Function Test: Remove the fuel injector for cylinder 5 and perform a flow test to confirm operation within specifications (30-50 lbs/hr). Check resistance (should be 12-16 ohms). If out of spec, replace the injector with an OEM part (OEM P/N 12576383).
- STEP 4 - Compression Testing: Conduct a compression test on all cylinders to assess mechanical integrity. Aim for uniform readings above 150 psi. If cylinder 5 shows significant deviation, further investigation into mechanical issues may be warranted.
