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Rough Idle & Misfires After DIY Spark Plug Change on 2018 G80 3.8L

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Mark S. · rep 0 · May 22, 6:22 AM · 1 replies · solved · Question

Vehicle: 2018 Genesis G80 3.8L

Hey everyone,

I recently decided to tackle a DIY spark plug replacement on my 2018 Genesis G80 with the 3.8L engine. It has about 75,000 miles on it, and I figured it was time for new plugs. I bought the recommended OEM-equivalent plugs and tried to follow a few online guides as closely as possible.

After putting everything back together, the car now has a noticeable rough idle, especially when cold, and feels a bit sluggish under acceleration. I also noticed the check engine light came on, and when I scanned it, I got a P0302 code, indicating a misfire on cylinder 2.

I double-checked all the coil pack connections and made sure the plugs were torqued correctly, but I'm worried I might have messed something up. Could it be a bad coil pack, or did I perhaps damage a plug during installation? Any advice on what to check next would be greatly appreciated before I take it to a shop.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 6:22 AM

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ClearTheCode admin answer:

It's common to encounter issues after a DIY spark plug replacement, especially if it's your first time. A P0302 code specifically points to a misfire on cylinder 2. Here are the most likely causes and diagnostic steps:

  1. Incorrectly Gapped or Damaged Spark Plug:

    • Cause: Even new plugs can sometimes come with an incorrect gap, or the gap can be accidentally closed/opened during installation. A drop or impact can also crack the ceramic insulator.
    • Action: Remove the spark plug from cylinder 2. Carefully inspect the ceramic for cracks and verify the gap using a feeler gauge. If it's off or damaged, replace it.

  2. Faulty or Damaged Ignition Coil/Connector:

    • Cause: The ignition coil for cylinder 2 might have been damaged during removal or reinstallation, or its electrical connector might not be fully seated. Sometimes, an aging coil can fail coincidentally after being disturbed.
    • Action: Swap the ignition coil from cylinder 2 with a known good coil from an adjacent cylinder (e.g., cylinder 1 or 3). Clear the code and drive the car. If the misfire moves to the new cylinder (e.g., P0301 or P0303), the coil is bad. If it stays on P0302, the issue is likely the plug or wiring.

  3. Wiring Harness Damage:

    • Cause: The wiring leading to the coil pack for cylinder 2 could have been pinched, cut, or disconnected during the process.
    • Action: Visually inspect the wiring and connector for any signs of damage or loose connections.

  4. Vacuum Leak:

    • Cause: While less common for a single cylinder misfire, a vacuum leak near the intake manifold for cylinder 2 could cause issues. Ensure all intake manifold bolts and vacuum lines disturbed during the process are securely reconnected.

When to See a Shop:

If you've performed the above checks and swaps, and the misfire persists on cylinder 2, it's time to consult a professional. They have advanced diagnostic tools to check fuel injector operation, compression, and more intricate electrical issues that might be beyond a DIY diagnosis. Continuing to drive with a persistent misfire can damage your catalytic converter.

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