98 454 Mag Mpi mid range missfire
 

Alright I’ve been reading on this forum for a while trying to figure out this problem.
just bought the boat from original owner 360 hours well maintained.
says he had the dealer do bigger injectors and other work new.
Boat idles great as soon as you get it around 1,000 rpms you can hear it breaking up, hold it wide open and after about 45 seconds it takes off like a bandit up to 4,000 rpms won’t go over. If you hold it mid throttle it rolls coal like a freightliner!
i did plugs cap rotor
changed fuel separator.
checked codes on engine none just flashes 12, watched 4 sets of 12 flash 3 times.
Also boat is a 260 Velocity
any suggestions or help is appreciated.

What ? and What ! and What. :)

you can not just throw larger injectors nor any injectors at it and leave the stock ecm and other internals of the engine. What is your fuel pressure ? what ms pulse width thru out the rpm range is the injectors showing ? You need a scan tool for real diagnostics,

my guess it is the injectors and fuel pressure

So what injectors are installed and the rating of those injectors ?

You need to post your engine serial number as well. Later 1998 was the changing of the big blocks for ecm and a few other systems

coil?

You need to find out exactly what was done.All Part #'s , reprogramming,and whom.

No assuming or you could be screwed.

Edit in: Did the engine used to have a Procharger or etc on it ?

my first question is when did this problem start.did it ever run right after the larger injectors were installed.the black smoke makes me think the injectors are causing the problem.