hi friends.. need some advice.
i own a 1998 american offshore 26 nx with a factory setup 454 mpi with a procharger.
boat starts up fine. then after about 5 minutes of driving, the rpm on idle goes to 2000 / 2100 . no matter what i do, it wont come down.
we did the same thing and left the motor running for 30 minutes out of the water , but wont replicate the same problem.
we put it in the water again today, let it warm up for 20 minutes dockside. rpm was beautiful at 1000.
then got in the boat to go for a ride and after 3-4 minutes on idle to get out of the marina area, shot up to 2000 and ran rough.
we changed the fuel pressure regulator with a new one last week, we went through the distributor,checked the advance, no leak on any vacuums, we are beat !

with the throttles down, how can it shoot up to 2000 ? maybe the map sensor or the ecu is sending info to the distributor to change the timing ? why ? basically, the only way rpm can rise on it's own without giving it throttle is through the timing changing...
but what is causing the timing to get a message to change and shoot up the rpm ?
we unplugged the map sensor and the engine dies out. we tested the temp sensor and unplugged it thinking it was sening a cold engine message causing the rpm to rise for warm up, but that didnt do anything, the sensor is working fine...the computer is a delphi/gm... i dont see a sticker of anything like arizona speed ....same for the map sensor. all seems like it is stock ?? again, mayobe i have another issue...anyone have any ideas ?

i sold the boat and was going to a 42 fountain... but this is throwing my new deal out.. i must get this resolved asap and deliver the boat to the buyer working as it should , or else, no 42 fountain !
help !

I have seen this before with the mefi system. It was the ecm that was at fault. no codes or anything just something wierd with the drivers or circuits inside.

Sounds like the [TPS] is telling the ECM to speed up on it's own, You will ned a scan tool and watch the TPS %s and the TPS volts to see what they do when it happens The volts need to be at .5 volts at idle and max 5.0 volts at WOT if they move on there own then the sensor, wire, or ECM is bad. You need to jump in there and start wiggling every wire and sensor you can find with it running to see if you can make it change rpms. And make sure all the ground and power wires/cables to the batt, and engine are clean and tight by wiggling them all. Here is the merc scan tool you need to find if you are trying to fix it your self, Good luck with.http://www.rinda.com/marine/marine.htm

Also you should have you timing light with a dial hooked up and ready to go when it does it because you want to see what the timing really goes up to or down to, I say that because your system when it is put in to service mode by the scan tool or by crossing the wires at the data link connector the engine will rise from idle any where up to 1600 to 2000 rpms on it's own when in service mode, So when it dose it, The timing IF set right should go down to say 8-10 normal degrees before top dead center, And I have seen them as hi as 12 deg, Or the timing will flare up some where as hi 34ish degrees, And if it goes up that would be considerd normal and if it goes down then you have a short some where from the data link connector to the modual inside the dist under the cap, Or it's the ECM. And by now you have pulled the cap and rotor and looked and changed out if needed right? Thay get a lot of mostior under there and could through this off also. I could see the I/C circket getting hot or loosing it's ground and throughing this off and maybe flaring the timing up also. the I/C circket is the modual under the cap and if it dose not have the white grease for heat under it that is not good. I say if you are in a hurry and don't have time for a shop to try to fitt you in, Install a new modual under the cap first then install a new [TPS] throttle sensor next. And if that dose not do it you got some mager testing to do still. But before you do all that wiggle every wire you can find. Hope this helps you some good luck, And remember this is all just a starting point for you. Not A sure thing. Here is a merc sevice manual to look at it might help some And you might want to save this. When you open it up look on the right top cornor for main menu in red and open that up for more manuls, Or just scrol down and look in the black numbers of the area you would want to read and open them up.http://www.boatfix.com/merc/Servmanl/16/16cov2_1.pdf

It sounds to me that the IAC is staying open. Once you advance the throttle to get the boat on plane, the IAC opens all the way. When you come off plane, the IAC is probably still open. On your engine, the IAC is under the upper plenum next to the throttle blades. Take it out and spray it with some carb cleaner and remove the carbon. Also clean the passages inside the cavity that it sits in. You should be able to move the plunger in and out. It will be hard to move and will not move freely, but it will move. If in doubt, change it.They are only about $70. Hope this helps, Eddie.