I will follow previous advice and flush line but need to know where water will water shoot out from when I remove the valve and turn water back on. Out from the hole where e valve is? Will water run behind the wall? And should I remove the mixer and the cap or just the cap?

There should be two set screws, one on either side of the mixer they control how much cold and how much hot you let through so that you can set it so when you open the valve full you can have the perfect temperature. The hot usually comes closed from the factory. Have you tried opening that set screw?

Jack's referring to the screwdriver stops locater where the supplies connect to the valve. They both should be fully open.

where water will water shoot out from when I remove the valve and turn water back on. Yes it will if works like it should,

No! You will just have it on long enough to see if you have pressure to the valve.

should I remove the mixer and the cap or just the cap? Shut the water off to the valve Open it up and remove the cartridge. Check the inlet ports for blockage. Now turn the water back on for a moment to flush out the supplies. You should have a strong stream of both hot and cold coming out. Reassemble and test. Better now? Good luck, Tom