>... To prevent this happening again we installed a 2" PVC threaded pipe with end > cap that is removed when pumping out. A better way that I "should have" > installed would have been put a 2" pvc one way ball valve so that any vacuum > would open the ball valve and let make-up air in and keep the smell out. > This could also have a threaded end cap should the ball valve start leaking > and a hose wash would not have cleaned it out. Obviously the vent was > installed at the top of the tank. Oh...DEAR! All tank vents have TWO functions--to provide an escape for air in the tank displaced by incoming contents (water, fuel, waste) and to provide a source of air to replace tank contents as they're being drawn out of the tank by pump (fuel, water, pumpout, macerator). It apparently hasn't occurred to you that a vent that isn't allowing enough air to be pulled INTO a tank during pumpout isn't allowing enough air to ESCAPE to prevent the system from becoming pressurized. So that PVC pipe and cap is a disaster just waiting to happen! If you're lucky you'll only get a face full of tank contents when you open the cap or the pumpout fitting. Or...sooner or later someone WILL attempt to pump out the tank without removing the cap...possibly a mobile pumpout service in the Keys that comes by when you aren't aboard...or a new guy...or you'll just get distracted... Murphy's Law requires it to happen. And because you've allowed that pipe and cap to replace necessary tank vent maintenance, you're gonna have a BIG problem. And there MAY be a third problem: CG regs require all tanks to be vented to the outside of the boat. I'm not sure your pipe and cap could pass an inspection. The right cure was very simple: straighten out the vent line, remove any filters and replace the vent thru-hull with one that will allow you to back flush it every time you pump out and/or wash the boat--and then actually back flush it every time. -- Peggie ---------- Peggie Hall Specializing in marine sanitation since 1987 Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and Other Sources of Aggravation and Odor" http://shop.sailboatowners.com/prod.php?51996