I have a 2003 496 efi that has a high idle especially on start up you put it in gear and it's like whoa nelly. I changed the iac valve only made a slight difference

It is likely a vacuum leak. Could be anything from a hose came off, cracked rubber, etc. In addition check throttle linkage for rigging.

when I close off the hose for the IAC valve it idles down to 600 will have to research further thanks for the response

Sounds like a bad iac. I would check the connector and than remove the iac and see if it is stuck open. Also check the iac gasket.

easy stuff first unhook the throttle cable then start the engine - - can you manual move the throttle blades to bring down idle rpms ?

Scan the engine and look at tps and IAC. Voltage on tps and PWM % on the iac -

IAC connector on top of

TPS issues

Agree possible vacuum leak and you can detect a vac leak by reading the scan of the engine

Dirty flame arrestor

AND what about the installing of a new IAC gasket and the IAC muffler ? is yours installed - the IAC muffler ? and if so how bad is it ? or did it suck itself into the hose ?

Hopefully you installed the IAC properly and in a 3 torque pass