Vehicle: Mercedes-Benz Vito 114 CDI Tourer Pro Auto
Registration: WP70 ASZ
Mileage: 160,665 km
Inspection Type: Vehicle Diagnostic Assessment
Customer Concern:
- Intermittent check engine light (MIL)
- Slightly unstable idle when cold
- No major drivability issues once warmed up
The Problem: Warning Light and Rough Cold Idle, But Drives Fine Warm
The customer reported an intermittent engine management light combined with a noticeably rough idle on cold starts. Once the engine warmed up, the vehicle drove normally with no loss of power under load.
This symptom pattern often causes confusion, as the vehicle appears “fine” most of the time, yet continues to log faults.
Initial Findings: Injector Adaptation Faults, Not Electrical Failure
A full system diagnostic scan was carried out with focus on the Engine Control Module (ECM).
The ECM had stored fault codes indicating:
- Fuel injector adaptation limits exceeded
- Faults recorded on injectors 1, 2 and predominantly injector 3
Crucially:
- No electrical injector circuit faults were present
- No intake air leaks were found
- Fuel rail pressure, DPF and EGR data were all within specification
This immediately suggested an injector imbalance or wear issue, not wiring, software or ECU failure.
Live Data Analysis: The Key Detail Others Miss
Injector correction values were analysed under multiple conditions:
- Cold start
- Warm idle
- Raised RPM
The data showed:
- Injector 3 exhibiting repeated excessive positive correction during cold start
- Correction values breaching the ECM’s stored adaptation limit early in cold operation
- Injector correction values stabilising as engine temperature increased
Once warm:
- Idle quality improved
- Injector corrections returned to stable, acceptable values
- No drivability issues were present
This pattern is classic for injector wear that is temperature-sensitive, not electrical injector failure.
Graph-Based Trend Analysis: Why This Matters
Graph logging confirmed:
- High-amplitude correction spikes on injector 3 during cold start
- Gradual normalisation of values as operating temperature increased
- Stable behaviour at warm idle and raised RPM
This confirms the ECM is adding fuel to compensate for an under-performing injector when cold, until its maximum learned correction limit is reached, triggering the fault code and MIL.
Injector Adaptation Limits Explained (Plain English)
Modern diesel engines constantly adjust injector fuelling to maintain smooth combustion.
When an injector:
- Delivers slightly less fuel than expected (often due to internal wear)
- Performs worse when cold
- Requires increasing compensation over time
The ECM records this as positive injector correction.
Once that correction reaches the ECM’s maximum allowed adaptation limit, a fault code is stored, even though the engine may still run acceptably.
That is exactly what occurred here.
Diagnostic Conclusion: The Real Root Cause
The evidence clearly confirmed:
- The intermittent engine light is caused by injector 3 exceeding its long-term adaptation limit
- The issue is mechanical injector wear, not electrical failure
- The fault is not software-related
- Replacing injector 3 alone is not guaranteed to resolve the issue permanently
The behaviour across all injectors suggests progressive injector wear, with injector 3 currently the most affected.
Why This Wasn’t an “Urgent Breakdown” but Still Matters
At present:
- The engine remains operational and drivable
- Performance under load is acceptable
- The issue is most noticeable during cold operation
However, leaving this unresolved can lead to:
- Increasing cold-start issues
- More frequent MIL illumination
- Worsening injector imbalance
- Potential starting problems in colder weather
Recommended Next Steps
To confirm repair strategy and avoid unnecessary costs, the following are recommended:
- Comparative injector leak-off testing (cold and warm)
- Further injector performance evaluation
- Targeted repair or refurbishment based on measured results
This ensures the correct injector strategy is chosen, rather than guessing.
Why This Case Study Matters
This job highlights several important points that many garages miss:
- Injector faults are not always electrical
- Adaptation limit faults point to wear, not wiring
- Cold-start data is critical
- Live data and trend analysis matter more than fault codes alone
- Replacing one injector blindly can be false economy
At Fault Fixer, diagnostics means understanding why the fault exists, not just identifying which code is stored.