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2018 Sierra 1500 5.3L Rough Idle & Misfire - Check Engine Light On

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Mark S. · rep 0 · May 22, 3:38 AM · 1 replies · solved · Question

Vehicle: 2018 GMC Sierra 1500 SLT

Hey everyone, I'm having some trouble with my 2018 GMC Sierra 1500 SLT, which has the 5.3L V8 engine. Lately, it's developed a really rough idle, especially when I'm stopped at a light. It feels like the engine is shaking more than it should.

To make things worse, the check engine light came on a couple of days ago. I had it scanned at an auto parts store, and it pulled a P0304 code, indicating a misfire on cylinder 4. I've heard about AFM issues with these engines, and I'm wondering if that could be the culprit.

Has anyone else experienced this with their Sierra? What steps did you take to diagnose or fix it? I'm hoping it's not something too serious, but the rough idle is definitely concerning.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:38 AM

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ClearTheCode admin answer:

The P0304 code indicates a misfire in cylinder 4, which is a common issue on the 5.3L V8 engines, especially those equipped with Active Fuel Management (AFM). Here's a breakdown of potential causes and steps:

  1. Common Causes:

    • AFM Lifter Failure: This is a very frequent cause. The AFM lifters can fail, causing a valve to stick open or closed, leading to a misfire. This often presents as a ticking or knocking noise from the engine. If a lifter has collapsed, it will directly cause a misfire.
    • Spark Plugs/Ignition Coils: A faulty spark plug or ignition coil for cylinder 4 could be the simplest cause. These are wear items and can fail over time.
    • Fuel Injector: A clogged or failing fuel injector for cylinder 4 can prevent proper fuel delivery, leading to a misfire.
    • Low Compression: While less common, a serious internal engine issue like a burnt valve or piston ring failure could cause low compression in cylinder 4, resulting in a misfire.

  2. Initial Diagnostic Steps:

    • Check for Ticking/Knocking: Listen carefully to the engine, especially on the passenger side (where cylinder 4 is located), for any unusual ticking or knocking sounds, which could indicate a failing AFM lifter.
    • Swap Components: If you're comfortable, you can try swapping the ignition coil and spark plug from cylinder 4 to another cylinder (e.g., cylinder 2). If the misfire code moves to the new cylinder (e.g., P0302), you've identified the faulty component. This is a good way to rule out the simpler issues first.
    • Fuel System Check: A fuel pressure test can help rule out general fuel delivery issues, though a specific injector problem might require more advanced diagnostics.

  3. When to See a Shop:

    • If swapping components doesn't resolve the issue or move the code, or if you hear any persistent ticking/knocking noises, it's crucial to take your Sierra to a qualified mechanic. AFM lifter failure can lead to significant engine damage if not addressed promptly. A shop can perform a compression test, leak-down test, and further diagnose the AFM system to pinpoint the exact cause. Ignoring a persistent misfire can damage your catalytic converter over time.
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