I have a Moen combo kit that was purchased from Home Depot and installed by my contractor. (We had a falling out so having them repair this will not work) They used PVC (CPVC) for the stub out on the tub spout. When we went to use the shower and the diverter was pulled on the spout it continued to spray a good amount of water. I called Moen who is sending a new spout out in case it was a diverter issue, however they said that the problem sounds like a backflow issue due to the use of PVC piping and it's interior diameter difference versus metal piping. I know the easiest option is to see if the new spout works when it arrives or get a universal that is compatible. However, if the issue is the PVC and this becomes an issue with any spout I use. Is it possible to use a copper connector/adapter at the end of the stub out to resolve the backflow issue? Or would I have to open the wall to replace the whole arm of the stub out with copper? I have no access to the back so if the PVC stub out is a serious flaw and a connector/adapter would not be an option then I am looking at ripping apart my new bathtub which I realllllly do not want to do.

I would wait and try the new spout before converting anything. This is most likely a slip on and the cpvc should get a thin coat of plumbers grease before you slip the spout onto the stub. I am not sure what the Moen tech was thinking but using a cpvc pipe rather than a copper will not cause and increase inpressure or volume so I feel its highly unlikely to be the problem.

You mentioned PVC and then (CPVC). They are 2 very different materials. Many new houses, mine foor one, has CPVC throughout with no issues. I have built 2 other houses with CPVC and have never had back pressure issues. CPVC has the same OD as copper and bit smaller ID but not to the degree that it should impede the flow. Sloppy plumbing practice CAN allow the cement to restrict flow. Remove the tub spout and put a flashlight back into the stub. What do you see? Make sure the shower head isn't clogged with debris from the plumbing process. Remove the head and turn on the water to flush the riser piping and note that the hole in the flow limiter is VERY small, particularly compared to the ID of the pipe. Replace the diverter, clean the riser and head and let us know what happens next.

Guess I should have read your post first, Brian

No problema, great minds think alike

It is CPVC that is why I indicated the specific material in parenthesis sorry about the confussion. The Moen rep talked about the Interior Diameter when she was trying to explain the backflow issue. I have not got the new spout yet.

I will take a look in the stub to see if there is any sloppy cement issues and check the shower head. There is very good flow to the head though seems like it has to do with the stub somehow but I will check it out tonight. Thanks!

the diverter was pulled on the spout it continued to spray a good amount of water

If that means that water is still coming out the spout when the diverter is pulled up, it is almost certain that it is a defective diverter.

If that means water is coming out between the wall and the tub spout, it means a defective O ring in the spout. Possibly damaged on installation. Could also mean glue on the exterior of the stub out where O ring fits. Carefully sand away.
If new spout does not solve the problem, before going into wall to replace stub out I would try wrapping stub out with one or two wraps of electricians tape to see is a good seal can be made by O ring.

Assuming a slip on tub spout, you cannot add an adapter to convert to copper. Not enough room.

It was the diverter. The new spout works perfectly. It probably was damaged on installation. Thanks everyone!

Glad we could help! Brian