I have a Formula with two 5.7 in it. The Iac valves are new and on the starboard side when performing the iac test I get no rpm change with the engine as I increase the %. When I go a -% the engine will stall, on the port side when increasing % the engine idles up and -% the engine will stall. What am I missing, is it a tps sensor, ecm issue?? Why is the starboard not responding like the port?
Please help!

If you did not buy the merc branded one (which really isn't made by them), check to make sure the other two holes are pluged. I've noticed a difference in the number of ports. Some of them have two other "unused" ports that can leak and cause enough air to pass that the computer will see no reason to issue commands to it.

To make it simple, pull them both off and see if the back sides of the iacs look exactly the same. I had exactly the same issue and this was my cause. Yours could be different but from what you're describing, either the iac is not working or there is enough air flowing that it's just not being called on.

leave the key on but engines off. Move the questionable iac valve and see if you hear it click or make noise as you drag the value bar or change numerical values...

You should hear a little bump from it when you move it to neutral or just past.

Or, if it's a worm gear drive and not a stepper solenoid you will hear obviously a different sound but none the less you can tell.