We had an issue with what was originally diagnosed as water in the fuel, engine would run for 5 mins then die. Mechanic tried to put tube down to bottom of tank to remove any water but found crud. Long story short, large hole cut out of stern above tank, about 5/6” of crud plus some chunks of steel which seemed to be the cut outs when the tank was installed just left rusting away.
Tank was vacuumed out, cleaned & a piece of steel welded in to close the hole. Sorted.