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Old 08-21-2016, 12:50 PM   #1
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toilet backs up when black tank 1/3 full

When my black tank is empty the toilet flushes just fine but once I get about 1/3 full I start getting bubbles coming up when I flush and at 2/3 full it is filling the pipe. I can break this apparent vapor lock by applying suction with a plunger and pulling air out of the the tank but the victory is short lived. I took it to the dealer and they cut the vent pipe and inserted a bore scope to assure patency. I have opened this divide(the repair was just a rubber slide over with clamps) just to see if that changed anything and it didn't. In fact, I still had to flush the toilet to get enough air flow to make it drain.

My last experiment was to put an electrical wire fish down to get a feel for the geometry and it seems that there is only an inch or so clearance between the end of the pipe and bottom of the drain. I don't know how deep the tank is but I suspect that this is my problem. Has anyone else experienced this? If so, how did you solve it?
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Old 08-21-2016, 02:17 PM   #2
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There was a thread a while back about the same thing and if I remember right the drop pipe was extended too far into the tank.

However from your description it sounds like you have a different problem. If you have to open the toilet to get the tank to dump, that leads to a plugged vent stack.

When they checked the vent, did they check it from top to bottom to make sure it was open all the way?
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Old 08-21-2016, 03:47 PM   #3
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toilet backs up when tank over 1/3 full

Not only was the vent open to the roof, today I took apart the splice and the vent was open just a few inches above the tank.

As I said, the toilet works fine and vents properly when the tank is nearly empty but above 1/3 full or so the toilet acts like it is not vented at all.
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Old 08-21-2016, 04:15 PM   #4
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Not a plumber but it sounds to me like the vent tube extends too far into the black tank. Once the liquid gets to that level about 1/3 full the vent tube is under water so no more venting for the tank.
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Old 08-22-2016, 12:01 AM   #5
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Yes it would seem to me that either the vent pipe or the down pipe from the toilet extends too far. I can't tell which. It appears that the tank is only about 6" deep so an excess of 3 to 4" would do it. With the vent pipe open near the tank I can hear the flush running with the tank more than 1/3 full which leads me to think that it is the down pipe. If it were the vent pipe the sound should mute when the end gets covered.
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Old 08-22-2016, 08:12 AM   #6
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Maybe an inspection camera would help figure out which it is. It's possible that it could be both.

Please come back and let us know what you find and how you fixed it.
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Old 08-22-2016, 12:55 PM   #7
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I have the exact same problem with my Mirada. There is a bend in the pipe between the toilet and black tank and sometimes, when too much toilet paper is used it blocks the entrance to the black tank. And air sometimes bubbles up and splashes water out of the toilet. I started using a softer tissue that shreds easily in water and I keep a 3-foot dowel handy to push the stuff down if it gets stuck.
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Old 08-22-2016, 02:28 PM   #8
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Have you considered taking the toilet off and seeing how far down into the tank it extends?
You said the vent pipe is clear, so the only thing left is the toilet itself extending into the tank. I don't know how this is possible. It isn't for a residential toilet, since there is no pipe from the toilet flange down. But with an RV toilet, it might have a pipe as part of the toilet that extends down from the base. If it were underwater, then that would certainly cause the problem you are describing.
Remember, air before water...
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