"The oil was overfilled. I removed half a liter of oil to bring the level down."""
that may explain the entire poor-running scenario...
yanmar's GMs and JH's are splash lube engines. this means that in addition to getting lube oil forced to the cylinder walls from the oil pump, this engine is also lubricating its cylinder walls by "splash lubrication," meaning that oil from the pan gets splashed up onto the cylinder walls when the engine is running. overfilling the splash-lube yanmar engines with oil is is VERY BAD . in this engine design , the oil in the oil pan splashes up to lubricate the cylinder walls. the engine would much prefer to burn pure diesel
with a proper air mix in its cylinders. with adequate lubrication of the cylinder walls from splash lubrication (ie the right amount of oil in the pan, measured on the dipstick to the 'full ' mark) the diesel fuel
to air mix is fine, and the diesel burns properly . when the oil is overfilled, there is excess oil in the oil pan. too much oil then splashes into the cylinders, and too much oil in the cylinders will adversely affect the ability of the engine to combust the diesel fuel
it is trying to burn. as a result of overfilling the oil in a splash lube engine your engine will now have to burn off that excess oil (unless you the owner take the excess oil out...).with too much oil in the pan, your fuel to air mix out of the injector is NOT set to burn this extra bunch of lube oil splashing up. so now in every power stroke of a given piston, there is extra lube oil on the cylinder walls that will either be combusted up with the diesel, or will drip back down into the oil pan. combusting too much splashed up lube oil in your cylinders will give black smoke and rough running. )so your poor engine is trying to burn the diesel using its known air-to-fuel ratio as set by the injector pump
, but its precision fuel-to-air ratio coming out of the injectors is now not working as well because there is excess oil splashing up which the engine has to combust . so the engine runs rough or not at all, until the excess oil is gone. so GET THAT EXCESS OIL OUT!! then your engine should run fine.
note also that how much oil is splash lubing is a function not only of the amount of oil in the oil pan available to be splashed, but also of the angle of heel of the boat. a steep heel makes more oil slosh up from the oil pan. an overfill of lube oil combined with a high degree of heel say over 17 degrees can lead to a very dangerous situation called ENGINE RUNAWAY. in this scenario, so much lube oil will splash onto the cylinder walls and be combusted that the engine RPMS suddenly shoot very high 4000+, you are now burning lube oil and diesel, the engine sounds like it's screaming, black smoke is pouring out, the more you heel the more you suck lube oil in and you can quickly destroy the engine. the remedy is immediately bring the boat to flat zero heel and drop the throttle to idle 800 rpms and hope to hell the runaway stops. you hope that these maneuvers brings rpms back to normal, because you CANNOT shut down an aluminum
engine running really hot using the stop solenoid or emergency
button. yanmar recommends idling your eninge for 3 to 5 minutes before shutting it down rapid immediate shutdown of a hot engine can seize the piston to the cylinder wall, then possibly bending a connecting road, scoring your cylinder walls, warping the head
, and other bad things...(well that's another thread we can start called YANMAR ENGINE RUNAWAY.. would you like to see some nice pictures i have of a bent con rod, scored cylinder, etc...not good. (and not my engine ..) )
you did not do an engine runaway to your engine . that's good...everything should be fine after you remove the excess oil.
moral of the story might also be : (1) check the companion thread on this forum on how to not 'overfill your yanmar' with oil when you are changing the oil and ...(2)
get a different mechanic