1998 Sunsetter Monsoon 320 Acting Up - could use some input

[Steve Prather]

Hey folks - would like some input/direction/ideas from your experience on an engine issue.  I'll be pretty detailed:

1998 Malibu with Monsoon 320 / Multiport EFI, had it since 2000 and been great.  Been running fine for most of its life (I put less than 25 hours a year on it now) but this past summer I made the mistake of buying gas on the lake and got water in my fuel.  After water got in the EFI ports the engine would not run or even crank.  Added some HEET fuel additive / water remover to the gas and was able to get it running but it ran really rough.  I replaced both fuel filters (pre-filter and pressurized filter) and added new 93 octane gas.  Boat cranked okay and idled but when running it was a little rough so I changed the spark plugs. After plug changes it runs great and has normal top end................BUT................now I have a weird problem............boat runs great but when I back off the throttle the engine dies - doesn't sputter or slowly quit but almost like you pulled the kill switch lanyard out.  It happens if I pull it back quickly or even try to ease it back.  The engine cranks right back up with no issue, takes off like a scalded cat (sorry PETA) and runs great 'til I back off the throttle and it dies every time.  I am at a loss - engine runs fine - could it be electronic - I don't know.  I do have an old Perfect Pass that I seldom use any more (did when I skied) but still operates - not sure if that could have any impact by malfunctioning, etc.  If someone has experienced this before or knows where I might search to see if someone has had this issue I would appreciate it.

Thanks!

[formulaben]

I'm going to guess your IAC is gummed up.  I'd take it out and clean it or just replace it altogether, as it's not that expensive.