"Black" tank does not hold much and pressurizes

[CptBacardi]

Not a fun topic I know, but all help greatly appreciated. 1989 Carver 38 Santego. The sewage holding tank does not hold very much waste, it seems like only 4 gallons or so. I can tell when it's getting full because the crown head starts flushing and then kind of filling back up a little like there's pressure in the tank. When I go to pump out it's always a horrible experience. You have to release the waste cap slowly and let the "stuff" which is under tremendous pressure leak out. If you take the cap all the way off it's like a fire hydrant of nasty shooting up into the sky. I've tried blowing into the two vent lines, but can't really tell if they are clogged or not. There was a pungent smell released when the blowing was ceased. Yea so this is no fun and I have no idea what to do. I've pumped out several times over the last couple of years always refilling the tank with clean water and pumping it out again 3 times. I've tried liquid Rid X for RV's. Toilet paper is never flushed. Nothing helps. There is a bit of a sewage smell at times coming from the berth. The tank can't be this small. What can I do??? Thanks!!

[tomschauer]

definitely sounds like the vent is clogged to start with. You should leak out of the vent before leaking out of the deck plate. Also sounds like you need to open the tank and verify it is not full of sludge that wont pump out.

[CptBacardi]

How do I clear clogged vent lines? Also, uhhh, you actually open the tank and check for sludge? The horror! Are there services for that? I don't know how I would clean out sludge...

[tomschauer]

find the vent line at the tank, remove from the tank and try some compressed air. Also, there may be a filter in the line for odors, it could be plugged.
I am sure you can find someone to clean out the tank or remove it and install a new one if it is full of sludge. Not a fun job. Best of luck.

[nightsky]

There is most likely a holding tank charcoal filter in the vent line (I'd be surprised if there isn't one). The filter could be blocked (replace filter) or the vent line could be blocked at the top of the tank if tank was overfilled and solids or toilet paper somehow got in the vent outlet or the vent hose could be blocked by insects at the vent outlet on the side of the hull. Easiest way to troubleshoot is to start by first finding the vent at the top of the tank, and tracing the vent hose outward to locate the filter and ultimately where the vent exits the boat. Try vacuuming out the hull vent fitting from dockside. If that doesn't do the trick then work backward toward the tank. Remove the hose from the vent outlet just inside the hull to see if tank pressure is relieved. If it does relieve tank pressure, then the vent fitting on the hull is plugged. If it doesn't relieve the pressure, then remove the vent line from the tank side of the filter (be prepared for effluent to come escaping from the pressurized tank). If that doesn't relieve the pressure, then remove the hose (carefully, with rags covering everything up to catch any effluent that could come rushing out of the tank) from the tank vent fitting. if that relieves the pressure then the hose between the tank and the filter is likely blocked. IF THAT DOESN'T WORK, then you'll have to try snaking something flexible in to the vent pipe to feel it is blocked or use compressed air to try to blow out any blockage. The reason I recommend starting at the outward extremity and working inward toward the tank is so that you might be able to avoid the messier aspect of this job. Removing the vent hose from the tank first will most likely relieve the tank pressure (ie, the tank vent fitting itself is not blocked) but will be maximally messy and still won't tell you where in the line the blockage is. Good luck with it.

[SanJuanDreamer]

I have a 98 Mariner 350.
I noticed the tank wasn't holding much and with 4 people on board, I emptied often.
I did the same... chemicals, rinses and multiple pump outs and all.
Using a strong flash light, I held to the side of the tank, I still couldn't tell how much was in there.
Finally, I removed the top to find about 53% of the tank was sludge gel....
I had to clean it out by hand.
Assuming there is no air filter, I would bet that same stuff got caught in the air vent too.

[mjk1040]

Since any work in this area can be a bit smelly, may I suggest getting some Vicks vapor rub or a similar product and placing the rub under ur nose on ur upper lip. This will over ride the tank smell so u can tolerate it during the cleaning process. It certainly sounds like ur plugged up and the entire system is going to have to be cleaned and cleared.

[CptBacardi]

Thank you all for the extremely helpful information and suggestions. I believe I have a handle on what to do now and will head down there tomorrow to do the necessary. I will be sure to update with findings. This is a great forum and I appreciate y'all. Hopefully I can help others here as well.

[CptBacardi]

Ok I looked at the tank today and found there is one vent line with one inline filter. No doubt it's clogged. I've noted some advice above to just install a new line with no filter. It would be easy to do, but my question is would that cause excessive odor to the port side through hull vent exit? Replace filter or just go with new line and no filter?

[km1125]

Source of the post Ok I looked at the tank today and found there is one vent line with one inline filter. No doubt it's clogged. I've noted some advice above to just install a new line with no filter. It would be easy to do, but my question is would that cause excessive odor to the port side through hull vent exit? Replace filter or just go with new line and no filter?

Depends on what's in the tank and how often you treat it!!

You can always just bypass it as a workaround and plan to replace it when you get a new one in. A little smell outside is much better than the risk of pressurizing the tank or having a vacuum distort the tank when you go to pump it out!!