I just bought this boat so going in a little blind. It started fine ran great with plenty of power, but but then it started to idle at 2000 rpms and would not back down. here is what I have done.
first it is a 2001 merc 7.4 mag mpi 385 hp
took the iac valve off cleaned all of the throttle body
after that surges then give it gas and sticks at 2000 rpms

replaced iac valve -no improvement
took throttle cable off just to make sure it isn't sticking but samething happening

butter flys are closed and all the air is pulling from iac it seems
I can feel the air difference at the iac when it is surging so think the computer is making this issue happen but I don't have a scanner.
so hoping for new ideas

Sounds like a possible vacuum leak.

^^^ could be a leak, but check the timing as well. The computer also uses timing control the idle.

Mine was a broken intake gasket (after replacing the IAC)

also, get a rinda or other scanner on that engine to see that the IAC is actually moving.. could be the wiring to the IAC, ignition as suggested, vacuum leak etc

going to check vacuum tonight that was my next step I agree I need a scanner I have one for everything else I own

The new IAC wasn't recognized unless you put the engine in service mode. I'd get the scan tool (or just jump it) into service mode before anything else. It could just be the new IAC hasn't/isn't calibrated.

how can I jump it into sevice mode?

I believe it would be pins A and B on the connector. One wire is black & the other is black with a white stripe. Those are the wires to jump when checking timing. I'm thinking vac leak as well if it is surging it sounds like the IAC is trying to compensate. Randy

Alright checked for leaks can't find any vacuum is 12-15 surge was better after I reset iac but still idles at 1200 then when you give it throttle it holds at 2000. Throttle positioning sensor?