toilet flushing/venting mystery

[Deleted member 113557]

I have a simple toilet, lav, shower bathroom. Lav and shower drain fine. Toilet does not. Each time - It flushes fine, but water either doesn't drain, or drains very slowly. (see video). Plunger doesn't do much. Have had plumber out to clear line 3 times in about a week. Last guy suggested toilet was old, not spraying enough water from rim, replace toilet. Made sense. So I replaced toilet. Same thing. He came back, spent about 2 hrs snaking, pulling toilet, scoping about 4' back into line. Nothing.

So it seems to be a venting issue. All 3 fixtures end up at one vent through roof. No big trees near vent at roof. Mostly pine trees anyway. Doubtful any animal got into vent, but maybe. And again, lav and shower share same vent and are draining fine. So since its only the toilet - if it's a dead mouse or something, it would have had to end up in the horizontal portion of vent between toilet and lav. Seems completely unlikely. Plumber gave up. I am on my own, or will have to keep trying plumbers ($$$). My next step was to get up on room with hose and run water into vent pipe?? But I don't think that will really reveal anything. Water will just go down unclogged portion of drain, right? I am at a loss. Tenants are getting irritated. Any help or ideas would be greatly appreciated -thanks!