New motor issues

No clues as to your car's engine, mods EFI or carb. That will make troubleshooting your problems more difficult.

That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.

The crystal ball says maybe EFI, so here goes the EFI route...

See "Help me create the Surging Idle Checklist" at http://forums.stangnet.com/showthread.php?t=534062
for help with your EFI idle problems. The first post contains all the updates to the fixes. I continue to update it as more people post fixes or ask questions.

One of the first steps in the surging idle sticky is to dump the codes.

I suffered a vacuum leak under the lower manifold on a new engine. I used Felpro race gaskets (I think they were pn 1250) and there was a mismatch with my iron GT40 heads and stock lower manifold. The gaskets were intended for larger ports and sucked air from under the manifold. Symptoms were idle hunting and a high soft idle that would not snap back to attention when you got off the gas. A guy spotted the leak by looking for vacuum at the oil filler neck. I switched to regular Felpro stockers and the problem was solved.