2018 Silverado 1500 V8 Misfire P0304 and Rough Idle

ClearTheCode admin answer:

A P0304 code specifically indicates a misfire detected in cylinder 4. While AFM can contribute to engine issues, a single-cylinder misfire typically points to a more localized problem. Here's how to approach it:

1. Check the basics:

   • Spark Plug: The most common cause. Inspect the spark plug in cylinder 4 for fouling, damage, or improper gap. A worn plug can cause a weak spark.
   • Ignition Coil: The coil pack for cylinder 4 might be failing. You can often swap the coil pack from cylinder 4 with one from an adjacent cylinder (e.g., cylinder 2) and see if the misfire code moves to the new cylinder. If it does, the coil pack is faulty.
   • Fuel Injector: A clogged or faulty fuel injector for cylinder 4 won't deliver enough fuel, leading to a lean misfire. This is harder to test at home but can sometimes be identified by listening to the injector with a stethoscope or by checking fuel trims with a more advanced scan tool.

2. Inspect for vacuum leaks: A vacuum leak near cylinder 4 could also cause a lean condition and misfire. Listen for hissing sounds or use a smoke machine if available.

3. AFM considerations: While less likely for a specific P0304, a faulty AFM lifter or solenoid could cause issues. If the misfire persists after checking the above, and especially if it's accompanied by ticking noises, AFM could be a factor. However, usually AFM issues can affect multiple cylinders or cause more complex driveability problems.

When to see a shop: If you've checked the spark plug and coil pack without success, or if you're uncomfortable performing these diagnostics, it's best to take your Silverado to a qualified mechanic. They have specialized tools (like fuel pressure testers, injector testers, and advanced scan tools) to accurately diagnose fuel system issues, vacuum leaks, or deeper engine problems like compression loss or AFM component failure. Continuing to drive with a persistent misfire can damage your catalytic converter.