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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.
Explore More Mercedes Engine Misfire Issues
For a deeper dive into all misfire-related problems, visit our hub page: Mercedes Engine Misfire Issues – Causes and Fixes. You’ll find grouped case studies, step-by-step diagnostics, symptom–cause–fix tables, and prevention tips to help you resolve misfire problems quickly and effectively.
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
