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.

Sounds like timing and fuel mapping needs to be fixed, Id get someone in the boat that can tune that MEFI..........where are you located

NOT to sound harsh but lets ask what dealer - a marine certified dealer and of what brands ?? why would whatever dealer out of the blue just come up with the boat needs bigger injectors ?? IMO they sound like they know nothing about marine engine systems unless what was the reasons for bigger injectors and they have have that correct answer.

If I were you I would find out what exactly you have for an engine and set up - then find a real marine shop that can get you up and running correctly.

Posting up here without eyes - ears - hands and proper diagnostics tools is not the way to go about fixing your app. Total waste of time and money for you inwhich is throwing darts at a unknown dart board.for everyone.

. Your engine possible might have a totally mix match set up with totally wrong and systems installed that might never run correctly.. So what is stock parts on your app and what is not stock on your engine ? What ecm do you have ? Where did you buy this boat ? who did the engine - the past owner of the boat ? anyways good luck

You'll need all the part #s(broken record here) and then call,or better yet...visit a place that reprograms the MEFI ECU's.

All EFI engines (cars, boats, sleds, motorcycles,etc) ,when things are changed (especially cams/inectors.etc) needs to go thru this.

There are a few good programmers avail by phone but that is 2nd to having someone be able to do it in person. Hopefully someone here knows of one in our state.

borrow a intake manifold&carb and install it,you just might be pleasently suprised.

May sound crazy but have you verified your exhaust is good? Had an issue with the 454 in my Powerquest - misfired under light load in midrange rpm and seemed to clear up when wide open. Turned out midrange operation produced enough pressure for leaks that killed a couple cylinders, wide open it wasn’t really noticeable other than losing 200 rpm on top end. Would be easy to rule out in your case by looking at plugs and pulling risers and looking.

There is usually a tiny screen under the regulator in most fuel rails. I'd check there 1st. if clogged, the fuel will be deadheaded and will be at fuel pump deadhead psi.
Ck diagram to see it has 1 before you go in. :)

I would suspect that the problem is the regulator and or screen (item 16) on the cool fuel module (item 15).

https://www.offshoreonly.com/forums/...issfire-3.html

The upper one is another regulator (item 5) but is used as a damper, and there is no fuel return from the rail its plugged (item 11)

https://www.mercruiserparts.com/bam/.../31798/2690/90

https://cimg9.ibsrv.net/gimg/www.off...c219acf3ba.jpg

My thought is with pressures that high then the lower regulator is probably damaged

You have 2X the pressure needed, if its that high the injectors wont fire correctly

Can you post a picture of the motor and or boat

Is it a green and white named Yea Rite?

Vacuum is used to change the amount of pressure on the small end.
Need an air source, gauge and pressure regulator. Apply air pressure to the larger hole in your first pic. Slowly increase the pressure and the regulator should open around 43 psi.

Do yourself a favor and replace rusted parts whether or not they're working properly. That fuel pressure regulator hat is just waiting to rust through and cause problem, most likely on the water.

^^ Agree, would be a good idea

Also check for a blockage going back to the canister filter. If the line is blocked, pressure cannot be regulated. Need to find out why pressure exceeded limits. Was the screen clogged?

A clogged vacuum line wouldn't cause pressure to get that high. The vacuum just changes the pressure a few psi

so I'm assuming this isn't the boat being discussed. If it is I have a lot of insight.

No need to apologize and I'm really glad that is not the boat. Not that it's a bad boat actually was awesome but it is/was ProCharged same year approximate hours and other mods. It has been for sale for a while and I was hoping nobody did something dishonest. This is a tight group and we watch out for each other.
The modifications are really unique for a naturally aspirated boat and I hope you sort out the issue. It truly sounds like the wrong tune in the computer, there are so many variables it would almost be worth talking to someone who can look at the setup on it.

No, it is nit a valid test. PSi opens it. It is a fuel psi regulator. VAcuumjust changes the psi (ust a few psi) needed to open it .

The return on the regulator opens at the preset psi.Sp, if it's 40psi regulator,the regulator should be partially open just after that psi.