A fuel or water tank vent line clog is very unlikely...I've never heard of any that weren't the result of a kinked vent line. The holding tank vent is another story...insects are attracted to 'em, the fuel vent thru-hulls that builders use on all vents either have screens in 'em that get clogged by corrosion or dust or pollen, or tiny slits in the bottom that even salt from sea water splashes can block...a design that makes it impossible to backflush the line. Add waste spilling into the vent line if the tank is over-filled, or even due to heeling to all that and you have a tank vent that's guaranteed to become blocked, either at the thru-hull or the other end of it--that end of the hose and/or the vent fitting on the tank.
The easiest way to prevent a blocked holding tank vent: replace the "vent" thru-hull with an open bulkhead or "mushroom" thru-hull that you can put a hose nozzle against to backflush the line every time you wash the boat and/or pump out.
Spring is when mud daubers build their nests, and they LOVE holding tank vents! So one of the first things you should do every time you use the boat each spring is check the vent thru-hull for a mud dauber nest, even if you didn't find one last weekend 'cuz the li'l buggers can pack a vent line in a just a couple of days. You may have to use a screwdriver blade or ice pick to break up the nest before you can flush out the line.
Every boat owner is full of good intentions to check the vent and backflush it regularly, but nobody's perfect. So pay attention to how your toilet is behaving. If a manual toilet is getting harder to pump, or any toilet starts bubbling or "burping" when you flush, that's a good indication of a blocked tank vent...it means the tank is becoming pressurized, creating back pressure that prevents bowl contents from going anywhere. If you hear hissing in the toilet during pumpout, that's a definite indication that the vent is blocked 'cuz if the pumpout can't pull in any air through the vent to replace contents as they're pulled out, it's gonna try to pull in air from anywhere it can. Stop pumpout immediately and clear the vent line!
Hope this answers your question!
--Peggie
