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2018 Q50 3.0t Rough Idle and Misfire - Time for Spark Plugs?

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Rachel A. · rep 0 · May 22, 5:09 AM · 1 replies · solved · Question

Vehicle: 2018 Infiniti Q50 3.0t Red Sport

Hey everyone,

My 2018 Infiniti Q50 3.0t Red Sport, with about 75,000 miles on it, has recently started acting up. I've noticed a pretty consistent rough idle, especially when I'm stopped at a light. It feels like the engine is shaking more than usual, and I've also had the Check Engine Light come on a couple of times.

I scanned it with my OBD reader, and I'm getting a P0302 code, which points to a misfire on cylinder 2. I haven't changed the spark plugs since I bought the car, and I'm wondering if that's the most likely culprit at this mileage. Is it a straightforward DIY job on the VR30DDTT engine, or are there any special tools or tricks I should know about?

Could it be something else entirely, like an ignition coil or even a fuel injector? Any advice or experiences with similar issues on your Q50s would be greatly appreciated before I start tearing things apart. Thanks!

✓ Accepted solution

ClearTheCode Admin

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

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Hi John,

A P0302 code indicating a misfire on cylinder 2, especially with rough idle at 75,000 miles on your 2018 Infiniti Q50 3.0t Red Sport, strongly suggests an ignition system issue. While spark plugs are a common wear item, it's crucial to diagnose properly before replacing parts.

Here's a breakdown of potential causes and steps:

  1. Common Causes for P030x Misfire Codes:

    • Worn Spark Plugs: The most frequent cause at your mileage. Spark plugs degrade over time, leading to weaker sparks and misfires.
    • Faulty Ignition Coil: An ignition coil provides the high voltage to the spark plug. If it fails, the spark plug won't fire.
    • Clogged Fuel Injector: A dirty or failing fuel injector can prevent cylinder 2 from getting enough fuel.
    • Vacuum Leak: Less common for a single cylinder misfire, but a significant leak could affect engine performance broadly.
    • Low Compression: A more serious issue, indicating internal engine wear.

  2. DIY Diagnostic Steps:

    • Swap Ignition Coil: The easiest first step. Move the ignition coil from cylinder 2 to another cylinder (e.g., cylinder 4). Clear the code and drive the car. If the misfire code changes to P0304, you've identified a bad coil. If it remains P0302, the coil is likely fine.
    • Inspect Spark Plug: If the coil swap doesn't isolate the issue, remove the spark plug from cylinder 2. Look for signs of wear, fouling (oil, carbon, fuel), or damage. Compare it to a new plug or one from a healthy cylinder.
    • Consider Fuel Injector Cleaner: While not a fix for a completely failed injector, a quality fuel system cleaner can sometimes help with minor clogs.

  3. Spark Plug Replacement on VR30DDTT:

    • The VR30DDTT engine's spark plugs are generally accessible. You'll need a spark plug socket (typically 14mm thin-wall), an extension, and a torque wrench. Ensure the engine is cool before starting. It's recommended to replace all plugs at once if they're due.
    • Torque Specification: Always use the manufacturer's specified torque for spark plugs to avoid over-tightening (which can damage threads) or under-tightening (which can cause blow-outs).

When to See a Shop: If the coil and spark plug swap don't resolve the P0302 code, or if you're uncomfortable performing these diagnostics yourself, it's best to take your Q50 to a qualified mechanic. They have specialized tools to test fuel injector pulse, fuel pressure, and engine compression, which can pinpoint more complex issues accurately. Ignoring persistent misfires can lead to catalytic converter damage over time.

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