The smoke is black, I presume?
It sounds like it's not completely burning the fuel during the power stroke.
Does it keep cycling after it's been powered off (does it huff and puff like it's still on)?
Quote:
Dieseling (in the sense of engine run-on, and disregarding combustible gaseous mixtures via the air intake) can also occur in diesel engines, when the piston or seals fail due to overheating, admitting engine oil into the cylinder. A structurally failing diesel engine will often accelerate when the throttle is released, even after fuel injection is switched off.
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