Why does my kitchen sink smell really bad when my washing machine drains? There are no blockages under sink and I followed all washing machine installation instructions to the letter.
Please help its doing our heads in!

Does your sink bubble or gurgle when the washer discharges? Back to you. Tom

Does your sink bubble or gurgle when the washer discharges? Back to you. Tom
Yes it does and the soap bubbles come back up the plug hole, and when it does this that's when it smells. Giles.

yes it does and the soap bubbles come back up the plug hole, and when it does this thats when it smells. Giles.
You have a partial clog in your combination drain line. What happens is that the washer discharges with great force and volume and when that discharge hits the clop it bounces back sending a bubble of sewer gas ahead of it. That's what the bubbles and smell are. The solution would be to snake from the kitchen roof vent. Put out enough cable to reach the base and 20 feet more. Flush by cycling the washer. Good luck, Tom

I just want to agree with Tom. Additionally, since you have w/m connected to kitchens sink drain, you should cleat that pipe periodically. We usually try to run w/m drain directly to 3" sewer and never to kitchen sink pipe.

Thanks both that makes sense, however I have had all the pipes out from under the sink and cleaned and found no blockages. Kitchen sink drains via undersink u-bend to outside greywater trap which in turn feeds into sewer. There has been no problem with sink draining. Is it possible that w/m feed into udersink is just too narrow?

the soap bubbles come back up the plug hole, and when it does this that's when it smells. Giles.
This indicates a back pressure. It could be caused by a partial blockage the plumber missed or as you suggested, It could be possible that w/m feed into udersink is just too narrow? Your w/m could be discharging more volume then your pipes can handle. Good luck, Tom