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2018 Q50 3.0t Rough Idle & Misfire - Time for Plugs or Coils?

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Mark S. · rep 0 · May 22, 5:09 AM · 1 replies · solved · Question

Vehicle: 2018 Infiniti Q50 3.0t Luxe AWD

Hey everyone,

My 2018 Infiniti Q50 3.0t Luxe (AWD) has recently started acting up. I've noticed a rough idle, especially when cold, and sometimes it feels like it's misfiring under light acceleration. The check engine light came on yesterday and I pulled a P0301 code, indicating a misfire on cylinder 1.

The car has about 75,000 miles on it now. I've kept up with oil changes and basic maintenance, but I haven't touched the spark plugs yet. I know the VR30DDTT engine can be a bit particular.

Is 75k miles a typical interval for spark plug replacement on these engines? Could it be something else like an ignition coil, or even a fuel injector? Any advice on diagnosing this further before I start throwing parts at it would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:09 AM

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Hello Mark,

It sounds like you're on the right track with your diagnosis. A P0301 code specifically points to a misfire on cylinder 1, and at 75,000 miles, spark plugs or ignition coils are indeed common culprits for the VR30DDTT engine in your 2018 Infiniti Q50.

Here's a breakdown of what to consider and how to proceed:

  1. Spark Plug Interval: For the VR30DDTT engine, Infiniti generally recommends spark plug replacement every 60,000 miles under normal driving conditions. At 75,000 miles, your plugs are definitely past due and could be contributing significantly to the misfire.
  2. Ignition Coils: While spark plugs are the primary suspect, a failing ignition coil for cylinder 1 could also cause a P0301. Coils can degrade over time, especially with extended spark plug use, as they have to work harder to bridge the wider gap.
  3. Diagnosis Steps:
    • Swap Ignition Coil: A common diagnostic trick is to swap the ignition coil from cylinder 1 to another cylinder (e.g., cylinder 3). Clear the code and drive the car. If the misfire code moves to the new cylinder (e.g., P0303), then the coil is faulty. If it stays on P0301, the issue is likely the spark plug or something else specific to cylinder 1.
    • Inspect Spark Plug: If you're comfortable, remove the spark plug from cylinder 1 and inspect it. Look for signs of fouling, excessive wear, or damage.
    • Fuel Injector: Less common but possible, a clogged or failing fuel injector for cylinder 1 could also cause a misfire. This usually requires more advanced diagnostics, like fuel pressure tests or injector flow tests.

Causes of Misfires:

  • Worn or fouled spark plugs
  • Failing ignition coils
  • Vacuum leaks
  • Faulty fuel injectors
  • Low compression (less common, but serious)

When to See a Shop: If swapping the coil doesn't isolate the issue, or if you're not comfortable performing these diagnostic steps yourself, it's best to take your Q50 to a qualified mechanic. They have specialized tools to test fuel injectors, check compression, and perform more in-depth diagnostics to pinpoint the exact cause of the misfire. Addressing a misfire promptly is important to prevent potential damage to your catalytic converter over time.

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