I just replaced an old pump with a Flotec FP4112. The pipe from the well comes out of the wall where there is a check valve (just replaced). The well pipe then goes through two 90 degree turns before connection to the pump.

The pipe exiting the pump starts with a 90 degree elbow directly connected to the top of the pump (which I am wondering whether that is my problem). It then turns 90, then I have a spigot on a T, and then it has a manual valve before going under the house to the spigots.

The pump runs fine when a spigot is open. When all spigots are closed, the pump shuts off and immediately goes into convulsions. The pressure switch continuously turns on/off/on/off with milliseconds between each on/off.

If I hook city water up to the spigot and pressurize the lines, the pump will stay off.

I've already exchanged the pump and replaced the check valve trying to solve this problem.

Any suggestions? Is this happening because I do not have a tank? Is it because I have a 90 elbow coming immediately out of the pump? Does it need to go up for some reason?

Thanks for any help.