I just put a new engine in. It's a 318 and it starts fine but won't lower the revs on idle. As when I rev it up, it comes down a bit but no where near normal idle.

 

New engine? As in new,rebuilt, used?????

 

verify throttle plate is closing up all the way. and that throttle cable is routed correctly letting it close up all the way.

next - with air filter removed and you staring down the throttle body, find the IAC motor and valve on the back of the throttle body. you can see down in there on the back.... verify electrical connector is plugged in. IAC pintle should close up at warm idle to choke off the little pin-hole in the bottom of the well. if it doesn't, replace IAC.

 

And an idea about how fast it is idling could be helpful also. Is it real high or just slightly?

 

could also be a vacuum leak.