7.4 idle surge

[loun]

2001merc 7.4 mpi long block installed into boat. Runs and performs great other than when reducing throttle back to idle. The rpm surges up and down for 5 seconds hunting for the correct idle speed, once it settles idle is smooth. Where should I look for the problem?

 

[alldodge]

2001merc 7.4 mpi long block installed into boat. Runs and performs great other than when reducing throttle back to idle. The rpm surges up and down for 5 seconds hunting for the correct idle speed, once it settles idle is smooth. Where should I look for the problem?

Pull, check and clean the IAC first then MAP and throttle position sensor.