* Injector: A clogged or faulty injector in cylinder 5 could be preventing proper fuel delivery. This is a very common cause of misfires. You can try a fuel injector cleaner, but often replacement is necessary. A fuel pressure test would help diagnose if fuel delivery is the problem.
* Compression: Low compression in cylinder 5 means the cylinder isn't effectively burning the fuel-air mixture. A compression test is crucial here to rule this out. Low compression could indicate worn piston rings, a blown head gasket, or valve problems.
* Vacuum Leak: A vacuum leak after the MAF sensor can cause erratic fuel mixture delivery and misfires. Inspect all vacuum lines and hoses connected to the intake manifold for cracks, breaks, or loose connections. Pay close attention to the area around cylinder 5.
* Wiring/Connector Issues: Though less likely after replacing the coil, double-check the wiring harness and connector to cylinder 5 for any damage, corrosion, or loose connections. A broken wire or poor connection could intermittently interrupt the spark.
* Camshaft Position Sensor (CMP) or Crankshaft Position Sensor (CKP): These sensors are crucial for ignition timing. A faulty sensor could lead to misfires, especially if the problem is only noticeable at idle. While less likely given your other checks, it's worth considering if other troubleshooting steps yield nothing.
Troubleshooting Steps:
1. Fuel Injector Test/Replacement: This should be your next step. Either a professional injector cleaning or replacement of the #5 injector is recommended.
2. Compression Test: Absolutely essential to determine if there's a mechanical issue within the cylinder.
3. Vacuum Leak Check: Visually inspect all vacuum lines and use a smoke machine (if available) to detect leaks.
4. Wiring Inspection: Meticulously examine all wiring connected to cylinder 5, including the injector and coil.
5. Check for Error Codes: Use an OBD-II scanner to see if any other codes have appeared.
Important Note: The fact that the problem is primarily at idle suggests a fuel delivery or vacuum issue more than a major mechanical problem like a severely worn piston. However, a compression test is crucial to rule out serious internal engine damage. If you're not comfortable performing these tests yourself, take your vehicle to a qualified mechanic. Diagnosing intermittent misfires can be tricky, and a professional can use their tools and experience to pinpoint the cause more efficiently.
