BMW fault 2B85 - Ignition timing adjustment (cold start)
P13EA · Efficiency deviation
2B85 is stored by the DME when the measured combustion efficiency at partial load during cold start deviates from the programmed target beyond an internal threshold. This fault is primarily a fuel quality indicator: the DME detects that the fuel's combustion characteristics are outside the expected efficiency window.
The DME calculates the difference between target and actual combustion efficiency during partial-load cold-start operation. When the integrated differential exceeds the threshold defined in the DME program map, the fault is stored. The service measure for this code is to replace the fuel.
- Module
- DME
- Severity
- Informational
- Likely cause
- Poor or contaminated fuel
- Common fix
- Filling with fresh high-quality fuel
Symptoms
- Subtle rough running or slight hesitation during cold start warm-up that disappears at operating temperature.
- Slightly higher fuel consumption during warm-up.
- The fault may be present with no noticeable driveability change.
Common causes
- Poor fuel quality: low octane, contaminated, stale, or high ethanol content that the DME's cold-start maps are not calibrated for.
- Fuel from a station with water contamination in the storage tank.
- Fuel left sitting for an extended period in a seasonal or stored vehicle.
Diagnosis
- Read the fault memory and check whether 2B85 is the only stored fault. If companion codes exist for fuel injectors, fuel pressure, or mixture control, investigate those first as the efficiency deviation may be a downstream symptom.
- Ask the vehicle owner about recent fueling: station used, fuel grade, whether the vehicle sat unused, or whether they filled from an unfamiliar station.
- Clear the fault memory. Fill the tank with fresh fuel at the grade recommended on the fuel filler cap. Run the vehicle through several cold-start cycles and monitor for recurrence.
- If the code returns with fresh fuel, check fuel pressure at cold start and inspect the injectors for leaking or poor spray pattern. Check for intake air leaks that could affect cold-start mixture.
Resolution
Drain the tank or burn through the current fuel and refill with fresh fuel at the recommended octane rating. Clear the fault memory. If the code recurs with known-good fuel, investigate fuel system hardware: fuel pressure, injectors, and intake air leaks. Address any companion fault codes found during diagnosis.