Perplexing Bubbling\venting issue with Toilet

[DIYPlumber]

Hello - I am new to the site and have a rather perplexing problem that has been plaguing me for 12 years now since we moved into this house. I apologize if this is long, but I feel it’s important that you see the entire picture because I had several plumbers look into this over the years.

My powder room toilet bubbles ONLY when the upstairs toilet directly above it is flushed. No other time will it bubble. This toilet is at the end of the sewer line.I have had the air vent cleaned, sewer line checked with a camera and a new toilet installed 2 years ago. There are no blockages in the system and everything flushes or goes down as expected. Just recently a plumber installed a mini vent between powder room sink and main sewer line thinking this was the answer - but no luck. My house was built in the 1950s with 1 roof vent and I have public sewer. I have no other clogg issues.

Sometime back as I was replacing the p trap on the sink just outside the powder room toilet I noticed when the trap was removed, the powder room toilet did not bubble. However I did feel a strong air flow coming out of the open trap when the upstairs toilet was flushed. Thinking it was a fluke, I reattached the trap and flushed the upstairs toilet again and the powder room toilet below bubbled. Just to be sure, I opened the trap and it did not bubble. I would permanently disconnect the sink trap but we do use it and the washer drains into this same drain pipe.

Any thoughts on my problem? I've seen these anti-vacuum traps from McAlpine (UK based company) but not sure exactly what it does or if it will release air from the trap to relieve some pressure. Then there is the issue of sewer odor if I do that. Any suggestions or ideas are much appreciated.

Thank you for your replies in advance.
DIYPlumber

 

[SHR]

Your problem is one of the most common problems. You have a partial blockage in your drain line downstream from your toilet. Snake the drain line to remove the clog.

 

[Mr_David]

Don't apologize for writing to much. Many posters don't provide enough information. An when they do, some responders don't read what you said.
I'm guilty of that sometimes.

I could be that the lower problem toilet was not plumbed and vented properly
I suspect the lower toilet is wet vented from the drain stack of the upper toilet.
when upper toilet is flushed, that slug of water passes the closet bend of lower toilet and causes the bubbling.

Have you been able to verify how the toilet is vented and or plumbed?

 

[Matt30]

Partial blockage will give you those symptoms like SHR said. Check out the venting as well.

I did a service call a few years back where the drain was clear but it was still bubbling, so I traced out the vents and I found that the vent from the downstairs bathroom, and he vent from the upstairs bathroom were twined together but they didn't vent outside, so it was just one big loop. The second I took the pipe apart it vented properly.