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2018 Sierra 1500 Rough Idle & Check Engine Light - Misfire Issues?

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Mike T. · rep 0 · May 24, 1:32 PM · 1 replies · solved · Question

Vehicle: 2018 GMC Sierra 1500 SLT

Hey everyone, I'm hoping someone can offer some insight into an issue I'm having with my 2018 GMC Sierra 1500 SLT, 5.3L V8. Lately, I've noticed a pretty consistent rough idle, especially when I'm stopped at a light or in slow traffic. It feels like the engine is shaking more than it should.

To make things worse, the check engine light has come on a few times. It's usually solid, but once or twice it flashed for a short period before going solid again. I managed to get it scanned at an auto parts store, and it pulled a P0304 code, indicating a misfire in cylinder 4.

I'm wondering if this is a common issue for this model year or engine. Are there any known TSBs or recalls related to misfires, ignition systems, or fuel delivery that I should be aware of? What's the typical culprit for a P0304 on these trucks? I'm trying to figure out if this is something I can tackle myself or if I need to schedule a visit to the dealership.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:32 PM

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ClearTheCode admin answer:

A P0304 diagnostic trouble code specifically indicates a misfire detected in cylinder 4. This is a common issue across many vehicle platforms, and for your 2018 GMC Sierra 1500 SLT with the 5.3L V8, it generally points to one of a few key areas. A flashing check engine light is particularly concerning as it signals a severe misfire that could be damaging your catalytic converter due to unburnt fuel entering the exhaust.

Here are the common causes and steps to consider:

  1. Ignition System Issues:

    • Spark Plug: A worn, fouled, or damaged spark plug in cylinder 4 is a very common culprit.
    • Ignition Coil: The ignition coil for cylinder 4 could be failing or faulty. You can sometimes swap coils with an adjacent cylinder to see if the misfire moves, confirming a bad coil.
    • Spark Plug Wire: If your model uses spark plug wires, a damaged or corroded wire can cause a misfire.

  2. Fuel System Problems:

    • Fuel Injector: A clogged, dirty, or failing fuel injector for cylinder 4 can prevent it from delivering the correct amount of fuel, leading to a lean misfire. Fuel system cleaners can sometimes help, but often replacement is needed.
    • Fuel Pressure: While less likely to be cylinder-specific, overall low fuel pressure could contribute to misfires, though this usually triggers other codes as well.

  3. Air/Vacuum Leaks:

    • Vacuum Leak: An air leak around the intake manifold gasket or a cracked vacuum hose near cylinder 4 could introduce unmetered air, causing a lean condition and misfire.
    • PCV System: A faulty PCV valve or hose can also lead to vacuum issues.

  4. Internal Engine Issues (Less Common but Possible):

    • Low Compression: Issues like a worn piston ring, damaged valve, or blown head gasket in cylinder 4 can cause a loss of compression, leading to a misfire. This requires more in-depth diagnostics.

When to See a Shop:

  • Persistent or Flashing CEL: If the check engine light continues to flash, drive as little as possible and get it to a shop immediately to prevent catalytic converter damage.
  • Lack of Tools/Expertise: If you're not comfortable performing basic diagnostic steps like checking spark plugs or swapping coils, a professional can quickly pinpoint the issue.
  • Complex Diagnostics: If basic troubleshooting doesn't resolve the issue, or if internal engine problems are suspected, a shop with specialized diagnostic equipment is necessary. They can perform compression tests, fuel pressure tests, and more advanced electrical diagnostics.

While there aren't widespread TSBs specifically for P0304 on 2018 5.3L Sierras, individual cases of faulty coils or injectors do occur. Your best first step is to methodically check the ignition components for cylinder 4.

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