The largest online repair guide library for every car

About us
Contact support
FAQ
Sign up
Sign in

2017 C300 Rough Idle, Misfire, and Check Engine Light

0

Angela R. · rep 0 · May 22, 4:22 AM · 1 replies · solved · Question

Vehicle: 2017 Mercedes-Benz C300 Base

Hey everyone,

I've been experiencing a really frustrating issue with my 2017 Mercedes-Benz C300 (W205 chassis) lately. The car has developed a noticeable rough idle, especially when cold, and sometimes even when warmed up at a stoplight. It feels like there's an occasional misfire, and the engine just doesn't sound as smooth as it used to.

The dreaded Check Engine Light (CEL) came on a few days ago. I haven't had a chance to get it scanned yet, but I'm hoping it's something relatively straightforward. I'm trying to figure out what common issues might cause this in a C300 of this vintage.

Has anyone else with a 2017 C300 experienced similar symptoms? What were the culprits? I'm looking for advice on what to check first or what parts might typically fail around this mileage (I'm just over 70k miles). Any insights on DIY diagnostics or common fixes would be greatly appreciated before I take it to the dealer.

✓ Accepted solution

ClearTheCode Admin

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

0

ClearTheCode admin answer:

It sounds like you're dealing with a common set of symptoms for the M274 engine found in your 2017 C300. A rough idle, misfire, and Check Engine Light often point to issues within the ignition or fuel system, or potentially a vacuum leak. Here's a breakdown of common causes and steps:

  1. OBD-II Scan for Codes: Your first step should always be to get the OBD-II codes read. Misfire codes typically start with P030x (e.g., P0301 for cylinder 1 misfire, P0300 for random misfire). These codes will help pinpoint the affected cylinder(s) and narrow down the possibilities.

  2. Common Causes for Misfires/Rough Idle:

    • Spark Plugs: Over time, spark plugs wear out, leading to inefficient combustion. Mercedes-Benz recommends replacement around 50,000-60,000 miles for many models. If yours are original at 70k, they are a prime suspect.
    • Ignition Coils: A failing ignition coil can cause a misfire in a specific cylinder. If your OBD-II scan shows a P030x code for a single cylinder, swapping the coil with an adjacent one can help confirm if the coil is the issue.
    • Fuel Injectors: Clogged or faulty fuel injectors can lead to an improper fuel-air mixture, causing misfires. This is less common than spark plugs or coils but still a possibility.
    • PCV Valve/System: The Positive Crankcase Ventilation (PCV) system, including the PCV valve and associated hoses, can develop leaks or blockages. A failing PCV valve can create a vacuum leak, leading to a rough idle and potentially misfires. This is a known issue on some M274 engines.
    • Vacuum Leaks: Other vacuum leaks in the intake system can also cause unmetered air to enter the engine, disrupting the fuel-air ratio and causing a rough idle.

  3. When to See a Shop:

    • If the OBD-II codes point to multiple cylinders misfiring, or if the issue persists after replacing spark plugs or a suspected coil.
    • If you suspect a fuel injector issue, as this requires specialized tools for diagnosis and replacement.
    • If the problem is intermittent and difficult to diagnose, or if you're uncomfortable performing the checks yourself.
    • Persistent CEL or flashing CEL (indicating a severe misfire that could damage the catalytic converter) warrants immediate professional attention.

For parts, always consider genuine Mercedes-Benz parts or high-quality OEM equivalents (like Bosch, NGK, or Beru for ignition components) to ensure reliability and proper function.

Menu

Help & account