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2021 G70 3.3T Rough Idle and Misfire - P0303 Code

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Mike T. · rep 0 · May 25, 4:34 AM · 1 replies · solved · Question

Vehicle: 2021 Genesis G70 3.3T

Hello Genesis community,

I'm having an issue with my 2021 Genesis G70 3.3T. Lately, I've noticed a rough idle, especially when the car is cold. It feels like a slight shake, and sometimes it's more pronounced. The check engine light came on, and my scanner shows a P0303 code, indicating a 'Cylinder 3 Misfire Detected'.

I've only got about 45,000 miles on it, so I wouldn't expect major issues this soon. I've heard about potential issues with ignition coils or spark plugs on some turbo engines, but I'm not sure if that applies to the G70.

What should be my first steps to troubleshoot this? Could it be something simple like a bad batch of fuel, or am I looking at something more serious? Any G70 owners with similar experiences, please chime in!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:34 AM

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ClearTheCode admin answer:

A P0303 code on your 2021 Genesis G70 3.3T signifies a misfire specifically in Cylinder 3. This can lead to rough idling, reduced power, and decreased fuel economy. Given your mileage, it's worth investigating a few common culprits.

Here's how to approach diagnosing this:

  1. Check Spark Plugs and Ignition Coils:
    • The most common cause of a single-cylinder misfire is a faulty spark plug or ignition coil. You can try swapping the Cylinder 3 ignition coil with an adjacent cylinder (e.g., Cylinder 1 or 5). If the misfire code then moves to the new cylinder (e.g., P0301 or P0305), you've identified a bad coil.
    • Inspect the spark plug in Cylinder 3 for signs of wear, fouling, or damage. While 45,000 miles is a bit early for plugs on some engines, performance-oriented turbo engines can sometimes wear them faster, or a single plug could be defective.
  2. Fuel Injector Inspection: A clogged or faulty fuel injector for Cylinder 3 can prevent it from getting enough fuel, leading to a misfire. This is harder to diagnose without specialized tools, but a shop can test injector pulse and spray pattern.
  3. Vacuum Leaks: While less common for a single-cylinder misfire, a vacuum leak near Cylinder 3's intake runner could lean out the mixture enough to cause a misfire. Listen for hissing sounds around the intake manifold.
  4. Compression Test: If spark and fuel delivery seem fine, a loss of compression in Cylinder 3 could be the culprit. This indicates an internal engine issue (e.g., valve problem, head gasket, piston ring) and requires a professional diagnosis.
  5. Wiring and Connectors: Ensure the electrical connector to the Cylinder 3 ignition coil and fuel injector is secure and free of corrosion.

When to See a Shop:

  • If swapping ignition coils doesn't move the misfire, or if replacing the spark plug doesn't resolve it.
  • If you suspect a fuel injector issue, as proper testing and replacement require specialized tools.
  • If you don't have the tools or confidence to perform a compression test.
  • If the check engine light is flashing, which indicates a severe misfire that can damage the catalytic converter, requiring immediate attention.
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