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Check Engine Light and Rough Idle: Mercedes-Benz S-Class 222 (M277 V12) Case Study
The Mercedes-Benz S-Class 222, powered by the M277 V12 engine, is known for its smooth, refined, and powerful performance. When this engine develops a rough idle combined with a check engine light, it can be worrying for owners who expect perfection from their flagship sedan.
A rough idle occurs when the engine runs unevenly at low RPM, producing noticeable vibrations. When paired with a check engine light (CEL), it typically signals ignition or fuel delivery problems. In this case study, we’ll look at how technicians diagnosed and resolved a cylinder 7 misfire in an S-Class W222 with the M277 V12 engine.
Vehicle Details
- Model: Mercedes-Benz S-Class W222
- Engine: M277 V12 twin-turbo
- Customer Complaint: Intermittent rough idle and illuminated check engine light
Customer Complaint
The owner reported:
- – Check engine light illuminated intermittently, then more frequently.
- – Rough idle noticeable when idling at traffic lights or in neutral.
- – Loss of refinement in performance, which became progressively worse.
The complaint suggested possible ignition or fuel-related issues, but a detailed diagnostic process was required.
Diagnostic Process
Step 1: Fault Code Reading
- – Using the Mercedes-Benz Xentry diagnostic tool, a quick scan was performed.
- – Finding: Fault code indicated cylinder 7 misfire.
- – Note: A fault code is just the starting point it identifies symptoms but not the cause.
Step 2: Live Data Analysis
- – Monitored actual values and fault counter for cylinder 7.
- – Continuous misfires confirmed the fault was present and reproducible.
Step 3: Ignition System Inspection
- – Checked the ignition coil connector and wiring for damage or corrosion.
- – No visible defects were found ruling out connector-related issues.
Step 4: Waveform Analysis
- – Used the Xentry Scope to measure ignition coil discharge.
- – Cylinder 7 waveform showed abnormal discharge patterns.
Step 5: Comparative Testing
- – Compared cylinder 7 waveform with a known good reference (cylinder 1).
- – Cylinder 1 displayed a perfect waveform.
- – Cylinder 7 showed a weak discharge signal, confirming coil malfunction.
Root Cause & Resolution
Identified Cause:
- – Ignition coil failure in cylinder 7.
Repair Steps:
- 1. Replaced the cylinder 7 ignition coil with a new OEM coil.
- 2. Cleared fault codes and reset adaptive values in the ECU.
- 3. Monitored live data no misfires detected.
- 4. Performed a road test to confirm smooth performance.
Result: The S-Class returned to its expected smooth, powerful, and refined operation.
Discover More Mercedes Misfire & Check Engine Light Solutions
For step-by-step guides on diagnosing engine misfires and idle issues, explore our Mercedes Engine Misfire Issues – Causes and Fixes.
To see all the possible reasons your warning light comes on, visit our Mercedes Check Engine Light – Complete Troubleshooting Hub.
Why Ignition Coils Fail
The ignition coil is responsible for transforming the battery’s low voltage into the high voltage needed to create a spark. In V12 engines like the M277, each cylinder relies on its own coil. When one fails, misfires and rough idle occur.
Common Symptoms of Failing Ignition Coils
- – Check engine light
- – Rough idle and vibrations
- – Engine misfires (intermittent or continuous)
- – Reduced fuel economy
- – Loss of power or hesitation
Causes of Ignition Coil Failure
| Cause | Explanation |
|---|---|
| Age and wear | Over time, coils degrade and lose efficiency. |
| Overheating | Heat damages the coil’s insulation and internal windings. |
| Electrical issues | Voltage spikes or wiring problems can cause premature coil failure. |
| Fluid leaks | Oil or coolant leaks can contaminate coils and lead to failure. |
What Causes Rough Idle at Low RPM?
Rough idle can result from multiple sources:
| Category | Possible Issue |
|---|---|
| Ignition system | Faulty spark plugs, coils, or ignition wiring |
| Fuel system | Clogged injectors, weak fuel pump, or dirty fuel filter |
| Vacuum leaks | Cracked intake hoses or gasket leaks disrupting the air-fuel mixture |
| Sensors | Faulty MAF, O2, or throttle position sensors sending inaccurate data |
| Engine mounts | Worn mounts amplifying engine vibrations, mistaken for idle roughness |
Symptom – Cause – Fix Table
| Symptom | Likely Cause | Fix |
|---|---|---|
| Rough idle + CEL | Ignition coil failure | Replace faulty coil with OEM part |
| Continuous misfire in 1 cyl | Coil or spark plug issue | Replace coil/plug, retest |
| Misfires in multiple cyls | Fuel delivery or timing | Inspect injectors, fuel pressure, timing |
| Rattling vibration at idle | Worn engine mounts | Replace engine mounts |
| Poor fuel economy | Weak spark or injector | Replace coils/plugs, clean/replace injectors |
Key Takeaways
- – CEL + rough idle usually means misfire: most often due to ignition or fuel issues.
- – Fault code ≠ root cause: advanced tests (waveform analysis, live data) confirm the failure.
- – Comparative waveform testing is powerful: faulty vs. good reference helps pinpoint the issue.
- – The M277 V12 uses multi-spark ignition: precision diagnostics are essential.
- – Always validate repairs: clear codes, monitor live data, and test drive before delivery.
Preventive Maintenance Tips
- – Inspect ignition coils regularly, especially on high-mileage V12s.
- – Replace spark plugs at the recommended interval to reduce coil stress.
- – Check for fluid leaks that may contaminate coils.
- – Use premium fuel to keep combustion clean.
- – Respond to CEL immediately delayed repairs can damage the catalytic converter.
FAQs on Check Engine Light and Rough Idle
Q1: What causes check engine light with rough idle in Mercedes S-Class?
Most often ignition coil failure, but it can also be caused by spark plugs, injectors, or vacuum leaks.
Q2: Can I drive with rough idle and check engine light?
It’s not recommended. Driving with misfires can damage the catalytic converter and reduce engine lifespan.
Q3: How much does it cost to replace an ignition coil in the M277 V12?
On average, €250–€400 per coil including labor. Since the engine has 12 coils, preventive replacement in sets is sometimes advised.
Q4: How do technicians confirm coil failure?
By comparing ignition waveforms from the suspected cylinder with a known good cylinder.
Q5: Can bad engine mounts cause rough idle?
Yes. Worn mounts won’t cause misfires but can create vibrations that mimic rough idle.
Author
Written by: Mercedes Expert
Automotive Technical Trainer & Mercedes-Benz Diagnostic Specialist
With years of hands-on experience repairing and diagnosing Mercedes-Benz vehicles, specializes in case-study-based troubleshooting guides that blend workshop accuracy with educational clarity.
Last Updated: September 2025
