Leakly coupler joint

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?
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It would help to tell us what kind of pipe, plastic/metal, diameter, etc!!!
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On Saturday, September 6, 2014 12:36:13 PM UTC-4, snipped-for-privacy@sbcglobal.net wrote:

If it uses a nut, they typically have plastic seals. New? Old? Many times when they leak it's because the pipes aren't aligned straight. Pipes need to line up, you can't rely on the fitting to pull together pipes that are off on a slight angle. Also, you want them to be hand tight, maybe a little more, as a starting point. If it leaks, you can tighten a little more, but they aren't intended to be made up real tight and oftern leak more if they are overtightened.
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snipped-for-privacy@fidalgo.net wrote:

Hi, IMO, this is wrong. DW installation manual mention this with drawings. DW drain should be connected to above P trap using Y connector and you don't need separate DW vent. If your DW vent is under the sink where the sewer smell will go? I could be wrong but that's what I think.
So there was

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On Saturday, September 6, 2014 3:43:26 PM UTC-4, Tony Hwang wrote:

I guess that's why he corrected it.

.
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Sounds as if the dishwasher "vent" is in reality an air gap that some jurisdictions require.
--
Mr.E

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