Vehicle: MINI Cooper S ALL4
Year: 2013
Mileage: 77,162 km
Inspection Type: Pre-Repair Diagnostic Assessment
Customer Concern:
- Engine misfire while running
- Poor and inconsistent engine performance
- Engine fault codes relating to combustion misfires
The Problem: Persistent Misfire and Poor Performance
The customer reported ongoing engine misfires accompanied by rough running and inconsistent performance. Fault codes relating to combustion misfires had been logged, raising concerns about possible injector failure, engine damage or compression issues.
These concerns are common with turbocharged MINI engines, where misfires are often misattributed to expensive mechanical faults.
Initial Assessment: Ruling Out the Big (and Costly) Worries
The vehicle presented in generally good mechanical condition for its age and mileage.
During the initial inspection:
- No wiring damage was found
- No injector faults were identified
- No fuel system malfunctions were present
This immediately suggested the issue was not related to:
- Fuel injectors
- Fuel pressure problems
- Internal engine damage
Diagnostic Evidence: What the Fault Codes and Live Data Showed
A full diagnostic scan of the Engine Control Module (ECM / DME) revealed:
- Multiple active misfire-related fault codes
- Combustion misfire detected on cylinder 1
- Misfires occurring across multiple cylinders
- Fuel injection deactivation triggered by misfire detection
Live data confirmed:
- Active misfire counts, particularly on cylinders 2 and 4
- No signs of injector failure
- No fuel pressure or mixture control faults
This data pointed strongly toward an ignition-related issue, not a fueling or mechanical one.
The Critical Test: Ignition Coil Swap
To confirm the root cause, ignition coils were swapped between cylinders.
Result:
- The misfire moved with the ignition coil
This is a definitive diagnostic indicator.
It confirms:
- The misfire is coil-related
- The cylinder itself is healthy
- Injectors, compression and ECU operation are normal
This single test ruled out thousands of pounds’ worth of unnecessary repairs.
Fuel System & Mixture Analysis: Why the Readings Looked Misleading
Lambda readings at idle measured extremely rich (value of 4).
At first glance, this could suggest:
- Over-fuelling
- Injector faults
- Sensor issues
However, diagnostic interpretation showed:
- Fuel delivery and injector operation were correct
- The rich mixture was a result of misfire, not the cause
When a cylinder misfires, unburnt fuel enters the exhaust, creating artificially rich readings. This is a symptom, not a fault.
Technical Conclusion: The True Root Cause
All diagnostic evidence clearly confirmed:
- Engine misfires were caused by faulty ignition coils
- Fuel system operation was normal
- No injector, wiring, ECU or compression faults were present
The ignition system was identified as the sole root cause of the issue.
Why Partial Repairs Are a False Economy on MINIs
Although individual coils may fail first, replacing only one coil or plug significantly increases the risk of:
- Repeat misfires
- Ongoing drivability issues
- Catalytic converter damage
MINI ignition systems are particularly sensitive to imbalance.
Engineer Recommendation
To restore correct engine operation and prevent further damage:
Immediate repair required:
- Replace all four ignition coils
- Replace all four spark plugs
These components must be replaced together to:
- Restore smooth and reliable performance
- Prevent recurring misfires
- Protect the catalytic converter and engine components
Partial replacement is not recommended.
What Would Have Happened Without Proper Diagnostics?
Without structured testing:
- Injectors may have been replaced unnecessarily
- Fuel system work may have been quoted incorrectly
- Engine damage may have been wrongly suspected
Correct diagnosis prevented misdiagnosis, wasted cost and unnecessary repairs.
Why This Case Study Matters
This job highlights:
- Why misfires don’t automatically mean injector failure
- Why lambda readings must be interpreted, not assumed
- Why coil-swap testing is essential
- Why guessing costs more than diagnosing
At Fault Fixer, diagnostics means proving the fault, not guessing the part.