I hate to say the injection pump or injectors, but it's possible.

After checking all of the above, check the fuel injection system.

Sa Injectors:
1. It's already defective.
2. Spray pattern is really bad.
3. Worst, it doesn't spray anymore.

Injection pump:
1. The pre-filter is clogged up.
2. The rotorhead [for rotary pumps] is already worn out. This gives the fuel the pressure it needs to be injected sa engine.

Doc Diesel