Diesel bug (fuel contamination) diagnostics and treatment

Diesel bug is microbial contamination (bacteria and fungi like Cladosporium resinae) that lives at the fuel-water interface in a diesel tank. Symptoms: black slime on the primary filter (Racor bowl looks like black jelly), rapid filter clogging (vacuum spikes daily), rotten-egg smell from tank vent, injector damage from corrosive metabolites. Prevention: (1) Keep tanks topped up in humid conditions (less air space = less condensation = less water for bugs to thrive). (2) Add a diesel biocide (Biobor JF, Killem, FPPF Killem) at every fill-up if you're in the tropics or the fuel sits more than a month. (3) Never leave water in the Racor bowl, drain daily. Treatment when contaminated: (a) Sample from the tank bottom with a bilge pump or stripping pump, if you see water and black stuff, you have bug. (b) Add a shock dose of biocide (5x the maintenance dose) and let it sit 24-48 hours. (c) Polish the fuel through a fuel polishing rig or an extra Racor plumbed as a recirculation loop, pump the tank through 2-micron filters until the return is clean. (d) In severe cases, drain and clean the tank mechanically, sludge doesn't dissolve. New biocide additions won't cure a tank that's already thick with slime, only physical removal will.