I have a big old Deutz engine installed in a 50 foot boat. Big old
Deutz 6FL714.

Now i had to travel for around 200 hours in the french canals - they
have a speed limit on the canals - as a result I always ended up
running the engine at 900 rpm (minimum recommended is 800 rpm). Even
at 900 rpm the boat was doing 9 km/h instead of the permitted 6 km/h...
I know I'm not supposed to run diesel engines almost at idle but I had
no other choice.

Note: the engine is air cooled so it can not be water.

Anyway: what happens now is the following.

Right cylinder bank: always no smoke from the exaust.

Left cylinder bank: at low rpm, always some white smoke (not much).
When the engine is run hot (1800 rpm), less white smoke. If i return to
low rpm, no smoke at all, then with the time (and engine cooling down)
the withish smoke returns.

At 1800 nice pieces of black soot where flying out of both the exausts
... and some black smoke too.

My idea is that running the engine at very light load "coked" the
engine. Is adding some "magic" additive in the diesel a good idea ? Or
better leave it as it is ?

This before i fork out money for compression tests and the like, the
engine is normally run at 1100 rpm or so and is almost never operated
to full rpms.

Cheers
Matteo