This problem sure does not represent the end of the world but it's proving to be a nuiscance. As most of us know, water vapor droplets accumulate in pvc vent pipes carrying LP gas furnace exhaust. I have a 2 or 3" pvc joint from an LP furnace stack directly above the furnace that is leaking a bit of exhust water at an elbow. I can just get up to the ceiling line and observe and perhaps work on the joint but it of course will be barely and externally only.

So, how does one plug a leak at a joint that is cemented previously but obviously not very thoroughly. I think I might just be able to open up the void slightly with a small drill. I was thinking the fix could then consist of picking a time when the joint is perhaps not leaking that badly and could it not entail possibly injecting some pvc pipe cement with perhaps a syringe? It may or may not also mean I might have to open the joint up a bit more with a small drill.

Any other suggestions out there to this nuiscance problem? Thanks in advance.

Bs5

It might be safer to cut it out and replace it. If it leaks water vapor, it could leak carbon monoxide.

Although dclynch's advice is the best way the easiest would be to clean the moisture away from the joint. Run primer around and into the joint.
Now slather PVC Cement all over the joint and poke some inside if you can.
That should stop the leak. Good luck, Tom

If Tom's way doesn't work, you can always use the left over primer and cement to glue in a new piece.