Md2030D fuel starvation?

[jac]

Coming back into the Hamble this afternoon and experienced something that I suspect is fuel starvation but would like the collective wisdom here as well.

Engine has done just over 20 hours on these filters and up till this afternoon, working fine. Tank is only about 1/3 full - last fill was last autumn. Fuel is treated with Marine 16. Boat is a Dufour Classic 36. There is a known issue with fuel tanks on these where due to the shape of the tank, 1/4 should be considered effectively empty. Tank does not have an inspection hatch so unable to check.

Engine started absolutely fine but revs rose and fell - maximum revs with WOT were massively down (rev counter is dead so exact figures unknown but seeemed to be about 1500 rpm)

Engine ran consistently so we came in without incident- just slow speed. After idling, the engine would achieve full revs but then die backdown to circa 1500. Looking at the bowl n the primary filter, it all looks clear but not changed the filters yet.

My view is this feels like fuel starvation. My assumption is that we may have picked up some diesel bug and that due to the tank being pretty low, one of the filters is partially clogged, limiting the flow of diesel. Drop the revs and the system catches up enabling full revs for a second or two but the blocked filter soon prevents any further high revs.

Plan is to change both filters next time I’m down, fill with fresh fuel and give plenty of biocide. And see what happens.

Anyone got any further thoughts/ am I missing anything?

 

[B27]

I don't think biocide is any substitute for making sure the tank is reasonably clean.
I would change the primary filter and slice open the old one to see what it's like in terms of clogging with crud.
Also I'd sample the bottom of the tank with a Pela, if your tank has the bottom 1/4 unusable, there could be a lot of water and crud in there.
I connected my pela to a length of 8mm bore hose with a weight on the end, that can be inserted via the tank filler.

You could have a fairly clean tank but bug growing in the filter.
But if the filters are only 20 hours old, that suggests the tank is dirty.

The best way to diagnose starvation is perhaps a vacuum gauge after the primary filter.
Until you start sucking air from the tank, fuel level makes no difference. Unless it's a very deep tank and a weak lift pump.

Dealing with the problem while the tank is fairly low can be good, it's easier to deal with 10 litres of suspect diesel than 100.
One boat I used to sail on, we removed about 8 litres of dirty fuel/water mix from the bottom of the tank, let it settle for a while in plastic milk bottles and eventually filtered about half of it back into the tank. The boat hadn't had the tank cleaned for some years and was OK for years after we cleaned the tank.

OTOH, you could have clean fuel and a lift pump or tank breather problem...