Usually suggests insufficient scavenge air supply, however, dependent on engine output it can be attributable to some other reasons.
- At or near full throttle indicative of inefficient combustion due to poor Ignition Quality resulting from high CCAI values.
- At low loads this is a sign of poor Ignition Quality. Increase scavenge air supply and coolant temperatures.
- At all outputs it is indicative of poor atomization usually due to high injection Viscosity. Raising final pre-heater temperature will reduce the viscosity at the point of injection, thereby, easing things.
- Mechanical failures resulting from fuel problems may also be to blame.
- The necessary scavenge air needed may not be getting to the combustion process due to turbo chargers fouling with coke formation.
- Atomization may be insufficient due to a lack of pressure resulting from fuel pumps / injectors wearing out (usual causes: Cat fines, Viscosity).