Toilet Mystery

[N80]

Whenever I have any sort of technical or mechanical issue I know I can come here and you guys will solve it.

I have a toilet mystery.

Background: I have repaired toilets many times. I have replaced toilets. I have installed new toilets and even the entire drain-waste-vent system in my cabin. And yet there are still things I don't understand about toilets.

My problem: 3-4 months ago our previously functioning master bathroom toilet (2nd floor) stopped maintaining the bowl water level. It flushes normally. The bowl fills back up to the normal level. But over 5-10 minutes the water level in the bowl (not the tank) goes down to about 1/4 of the normal level. I see no cracks in the bowl or waste channel. There is no water on the floor. There is no water or stains on the downstairs ceiling below the toilet.

It might be something obvious but I cannot figure this one out. Open to any and all suggestions. Thanks in advance.

 

[Peter 315]

Dogs?

 

[cholland]

is it vented properly? or is something obstructing the vent pipe? I have heard squirrels will sometimes fill vent pipes with acorns.
Poor venting, could create a vacuum and pull water down?

 

[MoKelly]

Other than dogs, there are 2 possible causes that I am aware:

1. Hairline crack in the tank allowing water to escape down the drain. You’d never see this on the floor or in the basement.

2. Clogged air vent. If there is a blocked vent, water drained on the first floor will result in drawing air from 2nd floor fixtures causing the water level in toilet to fall.

MoKelly

 

[MossRoad]

A vent pipe could be blocked or pitched incorrectly.

Or the toilet could have an internal crack in the S bend.

 

[N80]

Dogs? That's thinking out of the box and a good thought! Yes, we do have a dog but that's not it. You can watch the level go down.

Crack? I thought of that but I can't see how that is it. The S pipe on this toilet is visible. No visible cracks. For an internal crack to leak into the drain the crack would have to be in the upper part of the S and communicate with the lower part.

Clogged vent? Possible. However, when I had that problem before on another toilet the toilet would not flush properly because it could not draw air through the vent. This toilet still flushes briskly. Also, the vent on this toilet does not vent any first floor fixtures.

I'm not ruling any of these out except for the dog. The vent is on the roof. I'll have to have someone check it for me. It is nearly three stories high and a steeply pitched roof and I don't go up there. To find some sort of internal crack the toilet would have to be removed. I may do that if the the vent is not the issue. It will be hard to get myself to do that since it is otherwise working properly.

To be honest I'd thought of both of those but couldn't figure out how they could cause this issue. Certainly open to other thoughts or corrections in my reasoning (or lack thereof).

 

[Wagtail]

There could be a partial blockage at the bottom of the waste water pipe where it meets the 'main' pipe.

Due to the drop from the second floor, enough flushed water could be causing a slight suction (enough to claim 1/4 of the 'bowl' level) until the vacuum effect evens out.

 

[kennyd]

As mentioned above several times, I too suspect a vent issue.

 

[MossRoad]

At our current house we have a vent pipe that runs basement to roof. I found it had slipped in its brackets and dropped a couple inches. That allowed water to pool in the bottom of the elbow right below the vent vertical. I had to push it back up to correct pitch, then brace it permanently from the bottom to hold it up, because the brackets were buried in a wall I did not want to tear into. The issues caused by that situation were strange flushing(just didn't look like the normal swoosh), gurgling sink drains, and occasional sewer smell after gurgling events. Took a while to figure out, but visual observation and a level can tell you if pipes are pitched correctly.

 

[fidowanttobe]

Before you go to bed, put a drop of food coloring in whatever water is left in the bowl to see if it is still there in the morning. If it is not, you have a leak.