Toilet flushes except when sink is or has been on?

[RSQ]

Hi! I have a toilet on the first floor that has been exhibiting flushing challenges.

It's not clogged - I originally plunged it and blew out the wax seal (I've since bought a toilet auger), necessitating taking the toilet off and remounting it. While off, I took it outside and ran water through it. I then tried muriatic acid, which seemed to help.

However, now I've isolated a peculiar behavior. The toilet flushes fine, including multiple times in a row, except when the bathroom sink has been turned on. Then, it doesn't flush at all and water builds up in the bowl. Any ideas? Is this a vent issue?

The toilet is on the first floor, right above and at the end of the main PVC pipe in the basement that runs along the foundation and exits to the sewer on the other side of the house. The bathroom sink also drains into the pipe about two feet further down. I can leave the sink on for minutes without any issues in drainage.

Thanks!

 

[Reach4]

Sounds like an obstruction downstream of the sink. This presumes that the sink use involved a lot of water. However I cannot explain how the toilet flushes fine many times in a row. I am guessing that the fill speed limits having the toilet flush very quickly between flushes.

If you run the sink faucet at a slower rate, does the problem still occur?

It is not a vent issue.

 

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[RSQ]

Thanks for the fast response!

The sink does not seem to use a lot of water to me, as compared to other sinks in the house. Not sure if I was clear, but the sink drain connects to the main PVC pipe downstream of the toilet.

Regarding the toilet, I can flush the toilet every 30 seconds or so. It's a dual-flush toilet and I am testing with the half-flush setting, so maybe that's not too much water. However, I did perform the 5-gallon test on the toilet earlier and it all went down.

I should also mention that, once the toilet stops being able to flush (as it did just now), it stops being able to flush for a while. I haven't timed how long it takes for the issue to clear up, but I normally come back the next day and it works fine.