Plumbing Question

This sink is on a 2" line and no other drains are connected to it behind or near it. About 20' + it empties into the original main 4" black steel pipe that went to the old septic tank. I hooked that into the city sewer line in 1998. There is likely a trap on this line but def no vent pipe. It's been a while since I've crawled that far under the house and that ain't happening right now with my recent knee replacement! :D I am going to attach a rubber hose to where the DW attached to the white pipe under the sink and see if it works as a vent. If so then I have something that has changed with the DW. If not then maybe I'll install a snorkel as a vent, the wife would LOVE that! :D
I had to take some extra pain meds this morning and I just got back from PT so I am really not motivated to mess with the damn sink right now! But I will be diving in after I take care of a few things. Thanks so much for the input and I'll def report back.
 
R Q said:
maybe I'll install a snorkel as a vent, the wife would LOVE that! :D
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Click to expand...
A studor vent is cheap and easy to install, and can be placed under the cabinet. 99% of all residential island sink construction has these
 
R Q said:
About 20' + it empties into the original main 4" black steel pipe that went to the old septic tank.
Click to expand...

That changes things, and makes me skeptical that there's a venting issue that's suddenly changed that's causing the problem. Everything else dumps into the 4" line, I assume, and none of the other fixtures are slow or gurgling, etc...

Accumulation of sludge in the line that's causing slow drainage is still an option, but less of one.

How about a sag in the line? Is there a low spot on that 2" sink drain that could be causing the problem? Sludge will collect there and cause a restriction. If it's deep enough, it acts like a second trap and can trap air bubbles in the line that restrict flow.

Did you take the trap completely apart and make sure all those lines are 100% clear?

IME, on a 2" stub, the sink should drain super fast, even without a vent.
 
Update: With my knee issues I did not feel like chasing this anymore. Called a local plumber and he brought out a powered snake that is probably 2' longer than the one I rented! That's all it took. The line under the house could drain slowly so that's why I could pour a couple of pitchers of water into the pipe under the sink when I had it apart. Once I put all under sink back together, the trap couldn't work correctly I guess so it backed up quickly.
Anyway thanks for all the help and replies!
 
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