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The dishwasher drain line passes from the island, under the floor and back into the cabinet under the sink, connecting to the sink drain above the trap between the two tailpipes. There is no other plumbing beneath the island visible from the unfinished basement. The distance the drain line covers is just over 8 feet measured from the basement, the coil under the sink might be another foot and a half.

not to scale

I don't know how the dishwasher is connected to its drain or how much hose is concealed behind it, it's very awkward to extract from the island and I haven't worked up the courage to pull it out all the way.

The kitchen was renovated entirely for a previous owner, likely around three years ago. When we moved in earlier this month I thought the kitchen had a peculiar, somewhat unpleasant smell, particularly around the dishwasher. I may be used to it now; I have to really get close to the dishwasher to notice it.

My suspicion is that the length of the dishwasher drain below the floor is sitting filled with food grease from the dishwasher and this is what I'm smelling.

If this is indeed the problem, I imagine my best option (without renovating the kitchen) would be to run a separate drain with its own trap to the waste stack. Aside from getting in the way of finishing the basement one day, would there be any problems with that?

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  • If you add another trap it will not drain well , I think that may violate code , maybe try some bleach in the drain and see if that clears it out. I don’t see any thing wrong if the loop is required for a vacuum break.
    – Ed Beal
    Feb 26 at 4:37
  • Aside from the drain, dishwashers need periodic cleaning which require running an empty dishwasher with a cleaning product, such as Glisten Dishwasher Magic. If the previous owners didn't keep it clean, that could definitely cause it to stink. And read the manufacturer's installation guide to find max allowed drain length.
    – bsd
    Feb 26 at 8:37

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It's not totally clear to me if you've got this already just before your dishwasher hose enters the drain, but do you have a "high loop" in the dishwasher hose? This prevents water from draining from the sink down into your dishwasher drain hose and stagnating (see diagram below). If not, it should be easy to implement - just attach a loop of the dishwasher hose to the underside of the countertop right near the sink. It needs to be right near the attachment to the drain, not way over at your island dishwasher.

enter image description here

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