Originally Posted by Minnewaska
Our vent line exits the side of the boat. However, from the inside, you can see that it makes a loop up by the decks and then returns down to the fitting. I'm sure this is for anti-siphon. However, simply directing a hose spray from the outside, is not likely to make it to our tank. If I were to force the hose into the fitting, it would pressurize the tank, as the displaced air would have nowhere to go. The vent is blocked! Even opening the pumpout fitting may not work, as the intake is likely below the tank contents, so air will not escape. Be careful.
Like I said, while drawing a vacuum with the pump out, spray the water. This not only forces the water in (pressure of the hose) but it should be drawing air in through the vent (or trying to anyway). My vent is on the side of my hull as well and it fixed my clog like a champ.