For a long time I occasionally would smell sewer gas in the vicinity of my
kitchen sink. I thought it was because I hadn't been using my dishwasher. I
finally took a look under the sink and discovered that the discharge from
the dishwasher entered the drain downstream of the trap. So there was
nothing to prevent sewer gas from escaping from the dishwasher vent located
on the top of the sink. I revised the plumbing to correct this problem. In
one spot I had to use a coupler to connect two pieces of the drain and now I
have a slight leak at that joint. I've tightened the two nuts on either end
of the coupler as much as seemed safe to do but that didn't stop the leak.
What might be causing the leak and what could I do to correct it?