Electronics and water do not play well together. Pull the MPI crap off and toss it...then put on an Edelbrock Air Gap intake with a marine Holley 800 mechanical secondary carb and ALL your issue will be history.

i see you replaced the IAC valve. Did you clean the air passage for it that goes into the throttle body? Carb clean and piece of wire or something to stick in there to remove any gunk.

Try this,you have just about changed everything on the electronics end,,have you checked your charging system..try removing the + wire on the back of the alternator but make sure your battery is fukkt charged..start the boat and see how it runs..if the alternator goes bad sometimes it will throw a/c voltage @ the electrical system and screw everything up in the pcm's voltage..

Look for signs of fuel in the vacuum line between the plenum and the VST. You could be drawing in raw fuel under high vacuum/low fuel consumption conditions due to an overflowing VST. The line should be dry so check float and needle valves for wear/debris if it's not.

Worth a try,
GF?

GF just reminded me og somthing,,he brought up a good point about fuel..also check the fuel pressure regulator,when the diaphram ruptures in the regulator the engine will suck fuel in through the vacume port and fill the lower part of the intake..

Thanks for all the replies so far. Will start the elimination process again Fri. Have a schedule set up for scan tool and tech. early next week. Keep um coming.

What's the fuel pressure set at? Does a boat have a fuel return from the rail? if so, could it be restricted/plugged. Fuel Pump? Low voltage to fuel pump at idle.

Good Luck,
Bill

I had one do this exactly to me right after a rebuild. Changed the water circulating pump (bad) and it fixed everything. Now, who can tell me why that fixed it?

How did you know the water circulator was bad???

CTS cant read coolent temps if their is no water circulating.