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Rough Idle & Misfire on 2018 Q50 3.0t - Spark Plug or Coil Pack Issue?

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

Vehicle: 2018 Infiniti Q50 Red Sport

Hey everyone,

I've been experiencing a rough idle lately on my 2018 Infiniti Q50 Red Sport with the 3.0t engine. It's got about 75,000 miles on it now. Sometimes, especially when cold, it feels like it's stumbling a bit, and I've noticed a slight hesitation under acceleration.

The check engine light came on briefly a couple of times but then went off. I managed to pull a pending code P0303 (Cylinder 3 Misfire) before it cleared itself. I'm wondering if this is a classic sign that my spark plugs are due for replacement, or if it could be something more serious like an ignition coil.

What's the recommended spark plug change interval for the VR30DDTT engine? And is this something I can tackle myself in the garage, or should I take it to a professional? Any advice or similar experiences would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

It sounds like you're on the right track with diagnosing a potential misfire on your 2018 Infiniti Q50 Red Sport. The P0303 code specifically points to a misfire on cylinder 3, which is a strong indicator of an ignition system issue.

Here's a breakdown of what to consider:

  1. Spark Plug Replacement Interval: For the VR30DDTT engine (3.0t), 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 the primary cause of the misfire.

  2. Common Causes of Misfires (P030X codes):

    • Worn Spark Plugs: The most common culprit. Over time, the electrode wears down, requiring more voltage to spark, leading to intermittent misfires.
    • Failing Ignition Coils: The coil pack for cylinder 3 could be weak or failing. This is also common, especially as vehicles age.
    • Clogged Fuel Injector: A dirty or failing fuel injector for cylinder 3 could be delivering an incorrect fuel spray pattern or volume.
    • Vacuum Leaks: Less common for a single cylinder misfire, but a significant vacuum leak could affect engine performance broadly.
    • PCV Valve Issues: A clogged or faulty PCV valve can sometimes lead to oil consumption and spark plug fouling, contributing to misfires.

  3. DIY vs. Professional:

    • DIY: Replacing spark plugs on the VR30DDTT engine is a moderately challenging DIY task. It requires removing the engine cover, intake piping, and potentially some other components to access the rear bank. You'll need a torque wrench for proper installation. If you have experience with basic engine maintenance and the right tools, it's doable.
    • Professional: If you're not comfortable working on your engine, or if the issue persists after replacing the plugs, a professional shop can perform a more thorough diagnosis, including testing ignition coils, fuel injectors, and checking for vacuum leaks.

  4. Recommended Steps:

    • Start with Spark Plugs: Given your mileage and the P0303 code, replacing all six spark plugs is a logical first step. Use OEM or equivalent iridium-tipped plugs. While you're in there, inspect the ignition coils for any signs of cracking or damage.
    • Swap Ignition Coil: If the misfire persists after new plugs, try swapping the ignition coil from cylinder 3 with a coil from a known good cylinder (e.g., cylinder 1). If the misfire code then moves to the new cylinder (e.g., P0301), you've identified a bad coil.
    • Check PCV Valve: Consider inspecting or replacing the PCV valve, as it's a relatively inexpensive maintenance item that can impact engine health.

When to See a Shop: If replacing the spark plugs and swapping coils doesn't resolve the issue, or if you're uncomfortable performing these steps, it's best to take your Q50 to a qualified mechanic. They have specialized diagnostic tools to pinpoint fuel injector issues, compression problems, or more complex electrical faults.

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