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2017 C300 Rough Idle and Misfire Codes - What's going on?

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Mike T. · rep 0 · May 25, 3:05 AM · 1 replies · solved · Question

Vehicle: 2017 Mercedes-Benz C300

Hey everyone,

I'm having some trouble with my 2017 Mercedes-Benz C300 (W205, M274 engine) and hoping someone here has experienced something similar. Lately, the car has developed a really rough idle, especially when cold, and sometimes even at stoplights. It feels like it's shaking more than it should.

The check engine light came on, and when I scanned it, I got P0301 and P0302, indicating misfires on cylinders 1 and 2. I've heard these M274 engines can have some specific issues, but I'm not sure where to start.

Could this be spark plugs, ignition coils, or something more serious like an injector? Any advice on what to check first or common culprits for these specific codes on this model year would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:05 AM

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ClearTheCode admin answer:

Rough idle and misfire codes (P0301, P0302) on a 2017 Mercedes-Benz C300 with the M274 engine are common complaints. Here's a breakdown of potential causes and steps to diagnose:

1. Initial Diagnostics & Common Causes:

  • Spark Plugs: These are a common wear item. If they haven't been replaced recently (typically every 50,000 miles or so), fouled or worn plugs can cause misfires.
  • Ignition Coils: A failing ignition coil for cylinder 1 or 2 would directly cause the P0301/P0302 codes. You can sometimes swap coils between cylinders to see if the misfire code follows the coil.
  • Fuel Injectors: Clogged or failing fuel injectors can lead to an improper fuel-air mixture, resulting in misfires. A fuel system cleaner might offer a temporary fix, but a failing injector usually needs replacement.
  • Vacuum Leaks: Unmetered air entering the engine can disrupt the fuel-air ratio and cause rough idling and misfires. Check all vacuum lines and intake manifold gaskets.
  • Compression Issues: While less common, low compression in a cylinder due to worn piston rings or valve issues can cause persistent misfires. This requires a compression test.
  • PCV Valve/System: A faulty PCV (Positive Crankcase Ventilation) valve can cause vacuum leaks or oil consumption, leading to driveability issues.
  • Carbon Buildup: Direct injection engines like the M274 are prone to carbon buildup on the intake valves, which can restrict airflow and cause misfires.

2. When to See a Shop:

If basic checks like spark plug inspection or coil swapping don't resolve the issue, or if you're not comfortable performing these diagnostics yourself, it's best to take your C300 to a qualified Mercedes-Benz specialist. They have advanced diagnostic tools (like XENTRY/DAS) that can:

  • Perform a thorough scan for all stored fault codes, including manufacturer-specific ones.
  • Conduct live data analysis to monitor fuel trims, misfire counters, and sensor readings.
  • Perform a cylinder leakage test or compression test to rule out internal engine issues.
  • Properly diagnose and replace faulty components like injectors or address carbon buildup.
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