The laundry area was moved from garage to an addition at back of house (like that when I got the house). With that move, the washer drain pipe appears to share the same drain connection with the kitchen sink.

We've noticed that the kitchen sink drains slow when much running water goes through it, but not clogged. When the washer is draining, it can cause gurgling noises at the kitchen sink end, with a little sewer gas forced out. At times I think some bit of water backflows to the kitchen sink before draining out normally when washer is draining, but I haven't been monitoring lately to confirm. The gurgling noise & sewer gas for sure.

I'm writing this post to get ideas whether this combination of plumbing behavior is a sign of bad plumbing, partial clog in the system, or bad plumbing venting for this kitchen sink area. Plumbing at bathrooms seems to be find, just this kitchen sink area.

I don't know how the plumbing looks within the walls and up the roof, haven't taken apart the ptrap to see if there's any partial clogs along the sink drain piping, but assume not or very minor. But will check shortly.

Here's photo of the drain piping under sink. A plumber did mention the wall connection of the drain was not done to code when he came by to do some plumbing work. I do plan to fix that when remodel kitchen but not doing it yet.



Here's visuals of the kitchen drain pipes and what the drain connection look like at laundry area (when I first got the place, before I fixed it up). Laundry area is just in back of and to the left of the kitchen.

NOTE: the blue plastic valve is a water line connection from supply line for water filter & dispenser, supply lines behind the wall/board (old style wall mount kitchen faucet rather than the common sink counter mounted faucets).







Also, when the slow draining occurs, near the end, when water almost drained, it bubbles/gurgles with air coming out/up. And with that sometimes you get the sewer gas smell.

by the way, I don't see any air admittance valves installed (someone on another forum asked me about it).

I'm in California. Is the only way to fix this to open up the walls and redo the drain plumbing in the kitchen/laundry area? Not sure if need to add/rework the venting at the roof for the plumbing.

You probably have a partial clog in the drain pipe that is common with the kitchen sink and the laundry. You should have that section of piping mechanically cleaned. The trap arm(horizontal pipe between trap outlet and wall) rises up with a couple of fittings, where it should be pitched slightly downwards towards the wall. There is enough room under the sinks, to raise the tee and trap and replace the trap arm, so it pitches downwards.