Black stuff in fuel filters | TruckersReport Trucking Nov 26, 2008The primary cause of this black fluid is bacteria that loves to live in diesel fuel. This is algea that grows and is the source of the black you are finding in a filter. The ONLY cure for this is to get some biocide that will kill these critters. Racor makes a great biocide that you can get online. Thick black sticky sludge in fuel tank?! WTH - fiero Dec 23, 2011The black gooey stuff is the result of alcohol in the fuel.