If you have to keep spending those peanuts, not to mention the value of your time and effort, to keep it running, it doesn't sound to me like they're a very good investment. Everything has a lifespan...it's time to replace it unless you just want to keep throwing good peanuts after bad.
Should we discover that the vent line is plugged, what's the best way to unplug it? Water? Air?
You aren't gonna get out of this that easily...clearing a blocked tank vent requires some manual labor. However, like most things, prevention is a LOT easier than cure.
Vent line blockages typically occur in only two places--the vent thru-hull and the other end of the vent line...that end of the hose and the vent fitting on the tank. They rarely if ever occur anywhere else.
Start at the thru-hull...use a screwdriver blade, ice pick--whatever works--to scrape out anything that it's in it. If there's a screen, knock it out...screens create more problems than they solve or prevent. Then CAREFULLY open the cap on the deck pumpout fitting to relieve any pressure...you don't want to remove any hoses from the tank before doing this! Remove the vent line from the tank (warming the hose a bit with a blow dryer makes that easier)....scrape out whatever you find in either or both. Reconnect the vent line to the tank.
That SHOULD clear it. I also recommend that you replace the "vent" thru-hull with plain ol' bulkhead thru-hull--the kind you can stick your finger into...the same kind your sinks and bilge drain out of. This will allow you to prevent future vent blockages by simply sticking a hose nozzle up against it to back flush the line every time you wash the boat. Any critter that's taken up residence in it will just take a water slide ride into the tank. It's especially important to keep a close eye on the vent thru-hull in the spring, 'cuz that's favorite place for mud daubers to build a nest (I once had to clean one out of the venturi tube in the grill on my boat!)
NEVER overfill the tank! Waste builds up the tank fitting and that end of the vent line, creating a blockage. If you don't have a tank level indicator that lets you know how much is in the tank, install one!
Remove any vent line filter...they actually help to create the very problem they're sold to solve because they impede the flow of air in an out the vent line that's needed to keep a tank aerobic, which is the key to odor PREVENTION. Plus, they're expensive and are toast if they get wet--which makes it impossible to backflush the vent line to keep it clear. If your tank location or vent line installation makes impossible to use passive ventilation to keep the tank aerobic, some modification may work, or it may be necessary to install an aeration system. I'll be glad to help you work that out.
My thoughts: is it really that much easier to type a ' instead of a g ? Last I looked, the g is much closer to the n on a keyboard...
If a li'l ol' informal contraction upsets you enough to comment on it, my sloppy punctuation and slang contractions must give you chronic indigestion!
Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein