Fast idle problem with 2000 Z24

I recently purchased a 2000 Z24 Convertible with a blown engine. This past weekend we dropped a new engine in it. I feel we were very careful in making sure all of the wiring harness connections were done correctly. When I started it up, the motoe revved right to 3500 rpms, then it hunts around, going up to 4000, down to 2300, back to 3500, etc.

I put my OTC code scanner on it, and it does not recognize being connected to the computer.

Does anyone have an idea what would cause such symptoms?