Solve a fast idle

Hello, I've worked with the 3.0L V6 engine since 1996 in a Ford, and I can't ever recall one having a head gasket leak. The cast iron head and cast iron block kind of makes that very improbable. It would be more likely for you to have a cracked cylinder head -- which did happen on these 3.0L V6 engines -- than a head gasket failure. Beyond that, high idle is typically due to a vacuum leak, or an unmetered air leak into the engine. You'll have to check to ensure everything is connected properly and that all components utilized were proper, high quality (Ford?) parts for the vehicle and were installed correctly. It's so hard to come in after the fact and guess at what might be going on here, especially without performing hands on testing. Are you using a diagnostic tool to monitor engine performance criteria? Can you check fuel trims, MAF BARO readings? Is the exhaust manifold properly connected with no leaks? Is the throttle body properly installed? Is there an intake gasket leak? PCV system?