1. Ignition System:
* Spark Plug: The most common culprit. A fouled, worn, damaged, or improperly gapped spark plug on cylinder 5 will prevent combustion. Check for cracks, corrosion, and proper gap.
* Ignition Coil: The coil pack on the Ecotec engine often has individual coils for each cylinder. A faulty coil for cylinder 5 will prevent a spark. Visually inspect for cracks or damage. Testing with a multimeter is recommended.
* Ignition Wire (if applicable): While less likely with coil-on-plug systems, a damaged or loose wire from the coil to the spark plug could interrupt the spark.
* PCM (Powertrain Control Module): The PCM controls the ignition system. A faulty PCM can cause misfires by not sending the correct signal to the ignition coil. This is less likely than the other issues.
2. Fuel System:
* Fuel Injector: A clogged or faulty fuel injector for cylinder 5 will prevent fuel from reaching the combustion chamber. You'll likely need a fuel pressure test and/or injector testing to diagnose this.
* Fuel Pressure: Low fuel pressure across the entire system can cause misfires in multiple cylinders, including cylinder 5. A fuel pressure test is necessary.
* Fuel Pump: A weak or failing fuel pump might not deliver sufficient pressure.
3. Compression:
* Low Compression: A leak in the cylinder (valves, piston rings, head gasket) will result in low compression and misfires. A compression test is essential to check this.
4. Other Possible Causes:
* Vacuum Leaks: A significant vacuum leak can affect the engine's performance and cause misfires.
* Camshaft Position Sensor (CMP) or Crankshaft Position Sensor (CKP): These sensors are crucial for proper ignition timing. A faulty sensor can lead to misfires.
* Mass Airflow Sensor (MAF): A faulty MAF sensor can send incorrect information to the PCM, leading to incorrect fuel mixture and misfires.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting the spark plug, ignition coil, and wiring for cylinder 5.
2. Spark Plug Test: Replace the spark plug on cylinder 5 with a new one. If the misfire goes away, you've found the problem.
3. Ignition Coil Test: If replacing the spark plug doesn't resolve the issue, test the ignition coil for cylinder 5 with a multimeter or use a known good coil to test.
4. Compression Test: Perform a compression test to check for low compression in cylinder 5.
5. Fuel Injector Test: Check fuel injector operation (may require specialized tools).
6. Fuel Pressure Test: Test fuel pressure to rule out fuel delivery problems.
Important Note: Diagnosing a misfire requires systematic troubleshooting. If you're not comfortable performing these tests yourself, take your Alero to a qualified mechanic for diagnosis and repair. Incorrect diagnosis can lead to unnecessary parts replacements and wasted time and money.
