I have an above ground pool. The skimmer is fine when the pump is off but as soon as I turn the pump on, water starts to leak out of the bottom part of the skimmer along the wall on the outside of the pool.
I have had the gaskets replaced and the screws tightened on the skimmer and faceplate. What could be causing the leak?
The vibration caused by the pump running is just enough to start the filter leaking. That means it is a very minor leak because it stops when the filter is off. It should be close enough to being fixed that a little more tightening on the screws should get it.
Since you have already changed the gaskets you do not need to do that again. Hopefully you cleaned the wall of any left over gasket and rust deposits before they were installed.
You should closely inspect the way the gaskets are fitting and make sure the alignment is perfect. Gaskets out of alignment are impossible to seal. Loosening all of the screws enough to move the gasket just a little one way or the other is often enough to fix a skimmer leak.
If nothing seems to work and the leak is still there, try this. Get some silicone marine sealer, put it on your finger and lightly coat the skimmer area where the faceplate and wall meet. Depending on the skimmer, you may or may not have exposed sidewall and gaskets inside the skimmer area. If you do this method works every time. A small amount of sealer bonding the faceplate to the skimmer and covering the wall and exposed gaskets is all you need. If the faceplate comes into the skimmer opening and covers the sidewall it is a little harder to seal this way, but still possible. You need to work the sealer up into the area inside the skimmer, against the faceplate.
The last two options are ones you will probably not need to use. The entire skimmer could be replaced or your existing one could be removed and reinstalled. Before reinstalling coat all the different surfaces with silicone.
More information about leaking skimmers can be found here.
Above Ground Pool Skimmer Installation