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2018 C300 Misfire and Rough Idle - Check Engine Light On

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Robert C. · rep 0 · May 22, 4:13 AM · 1 replies · solved · Question

Vehicle: 2018 Mercedes-Benz C300

Hey everyone,

My 2018 Mercedes-Benz C300 (W205) with about 75,000 miles has suddenly developed a rough idle, especially when cold. It feels like the engine is shaking more than usual, and I've noticed a slight hesitation during acceleration.

Yesterday, the check engine light came on. I used a basic OBD-II scanner and pulled a P0302 code, indicating a misfire on cylinder 2. I'm hoping it's something simple like a spark plug or coil pack, but I'm worried it could be something more serious. Has anyone experienced this with their C300? What was the fix?

Any advice on troubleshooting this before I take it to the shop would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:13 AM

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ClearTheCode admin answer:

A P0302 code on your 2018 Mercedes-Benz C300 indicates a misfire specifically on cylinder 2. This is a common issue and can stem from several sources within the engine's ignition, fuel, or air systems. Addressing it promptly is crucial to prevent further engine damage.

Common Causes for Misfires (P030x codes):

  • Faulty Spark Plug: A worn, fouled, or cracked spark plug in cylinder 2 is a very common culprit.
  • Defective Ignition Coil: The ignition coil for cylinder 2 might be failing, preventing a strong spark.
  • Clogged or Faulty Fuel Injector: If the injector for cylinder 2 isn't delivering fuel correctly, it can cause a misfire.
  • Vacuum Leak: A leak near cylinder 2 could be causing an air/fuel mixture imbalance.
  • Low Compression: Less common, but internal engine issues like a worn piston ring or valve problem could lead to low compression in cylinder 2.

Recommended Steps:

  1. Swap Components: A good diagnostic step is to swap the spark plug and then the ignition coil from cylinder 2 with a known good cylinder (e.g., cylinder 1). Clear the code and drive the car. If the misfire moves to the new cylinder (e.g., P0301), you've identified the faulty component.
  2. Inspect Spark Plug: Visually inspect the spark plug from cylinder 2 for signs of wear, oil fouling, or damage.
  3. Check Fuel Injector: If swapping components doesn't isolate the issue, a professional can test the fuel injector's operation and spray pattern.
  4. Scan for Additional Codes: Use a more advanced scanner if available to check for any pending or historical codes that might provide more context.

When to See a Shop:

  • If swapping components doesn't resolve the issue or move the misfire.
  • If you suspect a fuel injector problem, as this requires specialized tools for testing and replacement.
  • If the misfire is accompanied by significant power loss, strong fuel smell, or excessive smoke from the exhaust, indicating a potentially more severe issue that could damage the catalytic converter.
  • For a comprehensive diagnostic, especially if you lack the tools or experience to perform the steps above. A Mercedes-Benz specialist can perform compression tests, leak-down tests, and advanced fuel system diagnostics.
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