have twin 496 ho's and port engine has been giving me some trouble.
#1 engine was running fine but stalls at 3800 -4000 rpm. would never pass 4000 while the other engine runs at 4600. changed plugs,changed low pressure fuel pump,fuel filter and still no luck. Just this weekend boat was working just fine minus the engine still wont go over 4000 rpm and engine started stalling and now runs really rough and loss almost all power. I swithed a few sensor from the starboard engine just to check and no luck, however when I unplug the MAP sensor engine runs smooth.I removed from the good engine and placed it and same results. Can someone please help me PLEASE!!

Without scanning the engine we would be all guessing. Just a guess---there are 2 fuel filters for each engine--might try changing both. The engine may be going into guardian mode and if so could be one of a variety of issues from sea water pressure to coils, fuel pressure, injectors, etc. Really need to get it scanned. Good luck....

thanks

You haven't mentioned any alarms. If you said you were getting the 2 beeps per minmute then full alarm once you hit 4k I'd say you had a bad IAC. Since you're able to hit the 4k mark it's nothing critical and the computer is robbing you of 10% power to self protect. Once you slow down to roughly 1100rpm the alatrm should go back to 2 beeps every minute. Try swapping the IAC's out too and see if the alarm/problem follows.

not getting any alarms. That's the funny part.

Had The Same 2 Wks Ago,was Told To Disconnecy The 2 Batteries And Clean All Terminal.changed Fuel Filter (no Water Found) .problem Gone!!

Thanks,
its worth a try.

no luck. same thing. motor has no power.

You need a scanner. If you don't have multiple issues going on, then the 4k limit is guardian.

MAP runs that engine for the most part. Everything falls back to it.

You can't guess this away. Must diagnose it.

Also, you need to know what the fuel pressure is under load. You said you replaced the lift pump. Did you check rail pressure which comes from the "other" pump??

Good luck!

Sounds like it is in guardian mode. Similar situation just happened to me. Idle air control (iac) failed. Would not idle in gear at all, idled rough out of gear. When it goes in guardian it basically floods engine with fuel and it wiped out all the plugs. Scanner is what you need unless you want to keep swapping parts. Iac is super common if you want to guess some more. You can put an ohm meter across the 2 terminals of the iac also and see if you get any reading.