Transparent P-Trap to help diagnose smell issue is wrong size

Question

What I've tried so far

I have this P-trap that I thought was standard. I bought a transparent replacement described as "Fits 1-1/2 in. or 1-1/4 in. ABS" but the fitting (reducing gasket) was too small. So I'm guessing that this is a non-standard size. In my searches I haven't been able to find a larger size that's transparent.

What I'm trying to do

I believe that I might have a siphon problem. In order to narrow down the problem I want to replace that P-trap with a transparent one so that I can see what's happening.

Looking for a solution that would help me get a transparent P-trap fitted onto this non-standard size coupling or some other solution that would help me understand if I have a siphon or other problem.

Problem is that it smells like sewer gas is coming back in. I've already removed and cleaned out this P-trap and replaced it once before but it doesn't make a difference and the P-trap was pretty clean and non-smelly itself.

I've read this answer and that might be what I need but that question seems to be a different issue.

P2000 · Answer 1 · 2023-01-27T22:20:30.153

Your likely problem is indeed improper venting.

You ask about ways to diagnose and confirm the problem.

When you have a foul smell, drain the P trap and measure the amount of waste water removed. You can catch it in a small container to see how much was in the trap.

If you don't like opening the unions of the one you have (it can be tricky and stiff at times), a transparent P trap would work of course, but also a "P Trap with Cleanout", a.k.a. a "P Trap with Drain Plug".

Image: Rona / Ipex.

If you have replaced the trap with the capped one, pour the water you drain from the new one in the old P trap to give you a quick idea of whether the trap is open or closed.

An AAV (Air Admittance Valve, a.k.a. cheater valve) under the sink will help, but you need to check your plumbing to determine the root cause. Your current dry vent may be clogged, and it may not be properly venting several other drains as well.

An AAV is not a substitute for circumventing a clogged vent.

They are allowed to assist venting a branch of your drain, but cannot be used in lieu of venting to the exterior. Any back pressure or methane build-up from the sewage will not be able to vent away through an AAV, and dangerous sewer gases can push through the P trap with sufficient pressure. Vents avoid this by relieving the back pressure.

Your vent stacks are likely visible on the roof, and you should inspect them to see if anything is stuck, like a pine cone, a clog of leaves or a rodent.