* Engine Design
* Defective Valve Or Seat
1. Engine design:
On diesel engines a small amount of soot is normal. This is due to the design characteristics of the engine. Valve overlap allows the intake to open slightly before the exhaust stroke has been completed, which will allow some soot to be pushed into the inlet manifold.
2. Defective valve or seat:
A defective valve or seat allows a large quantity of soot to be pushed into the inlet manifold.