running too rich ?

The throttle controls the fuel and the engine's turbo supplies appropriate air for the engine's speed.

Diesels can never run too lean as can gasoline engines.

Diesels will run too rich and smoke when their intake air is restricted by say, an oil soaked air filter.

A diesel's injection pump can be mis adjusted to inject too much fuel (smoke) and it can be mistimed to inject the fuel too soon (knocking), or too late (poor power and smoke).

A diesel with weak compression (due to worn piston rings and / or valves in need of adjustment) may smoke because its not achieving the compression and heat needed for good fuel combustion.

Hope this helps you.